Those are pretty big questions. 😸

Where do you live (general area)? That would affect your options of available brick'n'mortar stores and access to e-waste recycling centers.
What are the oldest and newest games you plan to play?
How much time and money do you plan to spend searching for your ideal system?
Do you have a specific hardware you like or dislike?

If you are still undetermined then I recommend watching Phil' s Computer Lab or his website for guides in addition to LGR.

Netherlands, Europe. But also don't have a car so I'm limited in my travel range. We do have our own regional eBay like used goods website, but they don't have a section specifically for older PC stuff, and it's kinda hard to find older parts in the ocean of current-day stuff.

Mostly want to play dos games and CD-rom games from the '90s era up until the early 00's. I'm not exactly in a hurry, but also don't want this to be a 5 year plan. It's gotta get made at some point. Don't really know specifics hardware wise, I was just a kid when Windows 98 was a thing, so I'm not too familiar with the technicals of that age. All I know is it needs to have a good sound card for both Windows and DOS, a 3.5 inch floppy disk drive, probably a 5.25 inch floppy disk drive, a DVD-rom drive, and probably a solution to use an SD card instead of a hard drive.

I'd recommend buying a finished (non OEM) system and modify it to your liking.

You mean a custom build by someone else? Those things are usually marketed as “a gaming PC for retro lovers”, including marked up price and I feel like it kinda defeats the purpose of building it myself.

I do have an old Hewlett Packard with Windows 98 laying around, but the quality is unsurprisingly unremarkable and the mobo doesn’t suit my needs.

JustRob wrote on 2021-10-17, 12:13:

Mostly want to play dos games and CD-rom games from the '90s era up until the early 00's. I'm not exactly in a hurry, but also don't want this to be a 5 year plan. It's gotta get made at some point. Don't really know specifics hardware wise, I was just a kid when Windows 98 was a thing, so I'm not too familiar with the technicals of that age. All I know is it needs to have a good sound card for both Windows and DOS, a 3.5 inch floppy disk drive, probably a 5.25 inch floppy disk drive, a DVD-rom drive, and probably a solution to use an SD card instead of a hard drive.

For sound cards, an ISA Sound Blaster (Pro 2, 16, AWE32, AWE64) would be the easiest choice: highly compatible and easy to find, but each generation and revisions has its own respective small defects; some better models are highly sought after. ESS AudioDrive is also a popular alternative.

That said, you need a motherboard with at least one ISA slot and, since you'd like to play some early 00's games, an AGP slot for a better graphics card is also necessary. This requirement limits to Super Socket 7, Slot 1, Socket 370, Slot A, and early Socket A motherboards (and their respective CPU). A combination of Socket 370 + Pentium 3 + GeForce 2/3/4 is very popular and not difficult to find.

A 3.5" floppy drive or a GoTek floppy emulator is always welcome and useful; a 5.25" floppy drive might not be necessary.

An IDE CD-RW drive would be more useful than a DVD-ROM drive: adds another method to transfer files, and they usually recognize recordable CD better than DVD-ROM.

An SD-IDE or CF-IDE adapter for main storage. Have at least two cards ready: one 2 GB with FAT16 for pure DOS, one 32 GB with FAT 32 for Win98SE.

Last but not least: look for local flea market, garage sale, or ask around friends and relatives to see if they have an unused computer collecting dust in the basement or attic.

JustRob wrote on 2021-10-17, 12:46:

I do have an old Hewlett Packard with Windows 98 laying around, but the quality is unsurprisingly unremarkable and the mobo doesn’t suit my needs.

Care to share its detailed specs?

I'm just sharing my experience as I quite recently dived into this hobby. At least in my area it is very hard to find cases from the 90s that doesn't already contain a system or they are about the same price. Sometimes AT cases turn up for sale with good prices but usually they are pick-up only for some reason. But maybe this is different in your area.

((Joakim wrote: I'd recommend buying a finished (non OEM) system and modify it to your liking.))

JustRob wrote on 2021-10-17, 12:46:

You mean a custom build by someone else? Those things are usually marketed as “a gaming PC for retro lovers”, including marked up price and I feel like it kinda defeats the purpose of building it myself.

Considering what is for sale on Marktplaats these days it is sound advice.
The parts that are in general on offer are more often than not overpriced and will turn out most likely more expensive than buying a complete working system and you can take it from there via upgrading. You miss the fun of the hunt but also the aggravation of being sent non-working crap which you then have to return.

When you select [Computers and software] on Marktplaats you can type 'retro' or 'vintage' in the box on the left which will trim down th‬e selection considerably.

search your area you live and can do pick up on the case, Like craigslist and facebook marketplace. Get a case first.

dormcat wrote on 2021-10-17, 12:54:

For sound cards, an ISA Sound Blaster (Pro 2, 16, AWE32, AWE64) would be the easiest choice: highly compatible and easy to find, […]
Show full quote

JustRob wrote on 2021-10-17, 12:13:

Mostly want to play dos games and CD-rom games from the '90s era up until the early 00's. I'm not exactly in a hurry, but also don't want this to be a 5 year plan. It's gotta get made at some point. Don't really know specifics hardware wise, I was just a kid when Windows 98 was a thing, so I'm not too familiar with the technicals of that age. All I know is it needs to have a good sound card for both Windows and DOS, a 3.5 inch floppy disk drive, probably a 5.25 inch floppy disk drive, a DVD-rom drive, and probably a solution to use an SD card instead of a hard drive.

For sound cards, an ISA Sound Blaster (Pro 2, 16, AWE32, AWE64) would be the easiest choice: highly compatible and easy to find, but each generation and revisions has its own respective small defects; some better models are highly sought after. ESS AudioDrive is also a popular alternative.

That said, you need a motherboard with at least one ISA slot and, since you'd like to play some early 00's games, an AGP slot for a better graphics card is also necessary. This requirement limits to Super Socket 7, Slot 1, Socket 370, Slot A, and early Socket A motherboards (and their respective CPU). A combination of Socket 370 + Pentium 3 + GeForce 2/3/4 is very popular and not difficult to find.

A 3.5" floppy drive or a GoTek floppy emulator is always welcome and useful; a 5.25" floppy drive might not be necessary.

An IDE CD-RW drive would be more useful than a DVD-ROM drive: adds another method to transfer files, and they usually recognize recordable CD better than DVD-ROM.

An SD-IDE or CF-IDE adapter for main storage. Have at least two cards ready: one 2 GB with FAT16 for pure DOS, one 32 GB with FAT 32 for Win98SE.

Last but not least: look for local flea market, garage sale, or ask around friends and relatives to see if they have an unused computer collecting dust in the basement or attic.

JustRob wrote on 2021-10-17, 12:46:

I do have an old Hewlett Packard with Windows 98 laying around, but the quality is unsurprisingly unremarkable and the mobo doesn’t suit my needs.

Care to share its detailed specs?

This seems pretty close to what I want. Maybe a SCSI adapter as well?

Sorry, it’s a Packard Bell, not Hewlett Packard. I don’t know if it’s completely original, it has product labels for both Windows ME and Windows XP, but I’m running Windows 98 on it. It’s a pentium 3. VIA integrated sound, a separate Ensoniq Creative ES1370 PCI sound card, a Nvidia Riva TNT2/TNT2 Pro AGP card (not in the best shape). It’s got a 3.5 inch floppy drive, both CD-rom and DVD-rom drives and a Maxthon hard drive. The motherboard says it’s a Micro Star MS-6323.

The case is beige with an ugly transparent greenish border bolted on the front, the keyboard that belongs with it is the same style. Not something I wanna use for this build, I think.

Don’t think there’s anything too useful in here.

JustRob wrote on 2021-10-17, 15:31:

Sorry, it’s a Packard Bell, not Hewlett Packard. I don’t know if it’s completely original, it has product labels for both Windows ME and Windows XP, but I’m running Windows 98 on it. It’s a pentium 3. VIA integrated sound, a separate Ensoniq Creative ES1370 PCI sound card, a Nvidia Riva TNT2/TNT2 Pro AGP card (not in the best shape). It’s got a 3.5 inch floppy drive, both CD-rom and DVD-rom drives and a Maxthon hard drive. The motherboard says it’s a Micro Star MS-6323.

The case is beige with an ugly transparent greenish border bolted on the front, the keyboard that belongs with it is the same style. Not something I wanna use for this build, I think.

I checked the spec of MS-6323. Sure, it's not the best Socket 370 MB out there, but with an AGP 4X slot along with your existing gears (any P3 CPU, TNT2, ES1370, DVD-ROM) I'd say you can still enjoy quite a few games of your choice. My first Win98 build was a P2-400 with the very first TNT graphics card (STB Velocity 4400) -- much weaker than yours -- and I played the original Rainbow Six series, System Shock 2, and Ultima IX for countless hours.

You just need a better case and a keyboard. 😉

I can’t get the ES1370 to work tho. Tried several drivers I found online, but no dice. And I dislike the lack of an ISA slot.

Also found another mobo that seems good, the AX6BC Pro. It has two ISA slots, five PCI and one AGP, but doesn’t come with an IO shield. Is it bad to build it in without one?

There are metal io shields that you can cut yourself. Finding an exact match is normally not that easy. 😀

JustRob wrote on 2021-10-18, 16:20:

Also found another mobo that seems good, the AX6BC Pro. It has two ISA slots, five PCI and one AGP, but doesn’t come with an IO shield. Is it bad to build it in without one?

An AX6BC Pro is better than MS-6323 for DOS games and ISA sound card support for sure. 👍 OTOH its AGP 2x slot would limit graphic card options for later Win9x games.

IMHO the IO shield isn't really important unless your environment is very dusty.

So it's a trade off, huh... I have a couple of unused Geforce 2 MX400 cards lying around, got them once in a box with parts... Does that work with an AGP 2x?

I found a GIGABYTE GA-6VXC7-4X-P, which I think is the mobo LGR used in one of his builds... It has only one ISA, but a 4x AGP port. I think that should be a better option for doing both Dos and Windows games.

JustRob wrote on 2021-10-19, 00:56:

I found a GIGABYTE GA-6VXC7-4X-P, which I think is the mobo LGR used in one of his builds... It has only one ISA, but a 4x AGP port. I think that should be a better option for doing both Dos and Windows games.

I have an exact same MB, paired with P3-800EB, 256 MB PC133 SDRAM, Radeon 9000 Pro with 128 MB VRAM (Gigabyte GV-R9000 Pro II), 60 GB HDD, and a Plextor CD-RW.

While I normally don't use it for DOS games (I've got another Pentium-MMX for those), it runs >90% Win9x games perfectly, many up to 1600x1200 resolution.

My 2 cents here:
Retro systems are like rabbits....
You build one and then you are surrounded by builds that always need "just one more thing".

Both of my recent systems have been built by Craigslist and OfferUp, as I'm always keeping an eye open for stuff people are just getting rid of. Complete P3 Win98 SE with a bad power supply? I'll take it for $15 if they are going to pitch it anyway.

EBay prices are just insane these days so I normally stay away...

Without a car I echo the same about finding a working system and then tweaking to taste.

I'm still waiting on that golden Voodoo2 find.... It WILL happen.

dormcat wrote on 2021-10-19, 02:43:

JustRob wrote on 2021-10-19, 00:56:

I found a GIGABYTE GA-6VXC7-4X-P, which I think is the mobo LGR used in one of his builds... It has only one ISA, but a 4x AGP port. I think that should be a better option for doing both Dos and Windows games.

I have an exact same MB, paired with P3-800EB, 256 MB PC133 SDRAM, Radeon 9000 Pro with 128 MB VRAM (Gigabyte GV-R9000 Pro II), 60 GB HDD, and a Plextor CD-RW.

While I normally don't use it for DOS games (I've got another Pentium-MMX for those), it runs >90% Win9x games perfectly, many up to 1600x1200 resolution.

Cool, looks like this should be the one then.

I'd love to make different PC builds for different types of games and OSes down the line, but it's mostly a space issue. Still living at home rn and I can barely fit one computer alongside all the other crap in my room.

JustRob wrote on 2021-10-19, 00:27:

So it's a trade off, huh... I have a couple of unused Geforce 2 MX400 cards lying around, got them once in a box with parts... Does that work with an AGP 2x?

Yes; the mobo in my 98/DOS only has a 2x and the MX400 runs fine