Id narrow the i/o card down to what io i need. since i dont know what your motherboard lacks its hard to say. If you need ide you should get a vlb card for sure.

a lot of people will say find a at adapter, and use a modern atx power supply. since your asking thats what id do if i wasn't sure what to get. also maybe more reliable and cheaper.

if you live in the US than ebay is your best bet.

PSU: If you take ATX supply with adapter, make sure you have reasonable amount of Ampers on 5V line. ATX type PC's started to use 12V rails more and their 5V lines may be quite weak.

MB: This is my personal preference, but a true 486 for me is non-PCI, but with VLB slots. And i would like to see as much L2 cache on board as possible. For future-proofing, it might be wise to try find a board that has voltage regulator on it as well, so you could later use 3.xV CPUs there. If you want to dig really deep, i would also recommend finding a board with chipset, that has MR BIOS available. 😉

IO: Unless you have any specific requirements, go for any generic VLB io controller. Why VLB? You may get a bit better throughput for your IDE drives. Everything else communicates still over ISA bus.

486 doesn't use that much power. Its like the socket A amds that are the worst and you need a beefy 5v rail for those. for 486 you are mainly lacking -5v on modern psus, some isa cards and motherboards use it. 20a on the 5v and 3.3v rail should be enough for 486

VLB motherboards tend to attract a premium, personally I'd just get what you can find for a good price rather then hunt out a specific make/model
Some motherboards will have I/O integrated but if not then VLB controller makes more sense then an ISA one, again find one for a good price and google it to see if you can find documentation on it, most importantly jumper settings.
Would be nice if the controller supports more then 512MB hard drives, alot wont, but not a deal breaker with overlay software.

Your choices for CPU, RAM, Sound are all good options. DX2/66 SB16 represents a mid 90's PC really well.
VLB video cards also attract a premium, more so then the motherboard. If you have a specific reason for wanting the Diamond fair enough. a few other brands like Cirrus logic make good fast cards but often overlooked by slightly faster brands but still offer just as good speeds in real life use.

Another option is getting a complete OEM system. 486 era used less proprietary parts then later systems and can form a good base for a slightly cheaper price then buying everything separate, Often the I/O and video were onboard on the VL bus and everything else was just stock standard hardware.

chinny22 wrote on 2021-01-07, 10:18:

Would be nice if the controller supports more then 512MB hard drives, alot wont, but not a deal breaker with overlay software.

Unless you have a controller with its own bios, this limitation comes from bios, not controller.
That's one of the reasons, why MR BIOS is so desirable.

chinny22 wrote on 2021-01-07, 10:18:

Another option is getting a complete OEM system. 486 era used less proprietary parts then later systems and can form a good base for a slightly cheaper price then buying everything separate, Often the I/O and video were onboard on the VL bus and everything else was just stock standard hardware.

Hmm, fair enough. Which OEM brand would be the most reliable?

Edit: Also, in terms of affordability, what would the best choice for a MIDI sound module (MT-32, SC-55, etc) be?

RetroLizard wrote on 2021-01-07, 16:52:

Hmm, fair enough. Which OEM brand would be the most reliable?

I wouldn't limit myself to brand especially as shipping will up the costs a fair bit. more on what's available locally.
in my case I've 2 PC's from brands I doubt anyone knows unless they used them in the day namely

Osborne Australia
Osbone 486 DX2 66 VL-Bus (My 1st PC ever)

and Tatung
I accidently got a TCS9510 486 VLB PC

I'd probably not go with Compaq, personally I really like the brand and build quality was really good but they did tend to things differently then everyone else.
Loke modern OEM PC's the motherboard or case wont work with generic parts so make sure you like the case it comes in, or be prepared for a bit of case modding.

Cheap midi doesn't exist 🙁
MT32 (and its variants) is its own thing more suited for early 90's like the Sierra titles, anything else wont sound right. It'll even be called this in the setup menu.
Later games supported General Midi which is a standard rather then a specific device. This is where you SC55, Yamaha, Dream Blaser, etc but even the not so popular choices like Korg aren't cheap anymore.
No matter which way you go you'll have to invest a bit of money here

1st gen Pentium provides best game play performance for DOS games.
Pentium 100mhz thru 233mhz.

Some DOS game play on a 486 is choppy. The CPU is not powerful enough.

So if you Goal is to play the most DOS games with good performance then get a Pentium CPU.
Also the 486 can NOT play MP3’s.

Intel486dx33 wrote on 2021-01-07, 18:44:

1st gen Pentium provides best game play performance for DOS games. Pentium 100mhz thru 233mhz. […]
Show full quote

1st gen Pentium provides best game play performance for DOS games.
Pentium 100mhz thru 233mhz.

Some DOS game play on a 486 is choppy. The CPU is not powerful enough.

So if you Goal is to play the most DOS games with good performance then get a Pentium CPU.
Also the 486 can NOT play MP3’s.

I'm not really looking for a 486 to play games with - looking for a 486 to make old DOS games with.

Also, what's generally the safest average number for a power supply? 150w? 250w?

And what are some of the better brands from back then?

I've had Seventeam and FSP branded PSU's that haven't required any maintenance yet. Though they were NOS, so you should at least (used or not) measure the voltage output and then check the insides for any visible trouble. Good thing is, that even the most basic PSU's from the early 90's used mostly Japanese capacitors so they usually haven't leaked yet.

Reasonable enough, I suppose.

(As a side note, (I don't know if this is allowed on the forums or not, so apologies in advance if it's not) if anybody has spare 486-era parts they don't want anymore, I'd be willing to buy it.)

If you're developing what kind of game are you working 2D or 3D?

2D

RetroLizard wrote on 2021-01-07, 19:05:

Intel486dx33 wrote on 2021-01-07, 18:44:

1st gen Pentium provides best game play performance for DOS games. Pentium 100mhz thru 233mhz. […]
Show full quote

1st gen Pentium provides best game play performance for DOS games.
Pentium 100mhz thru 233mhz.

Some DOS game play on a 486 is choppy. The CPU is not powerful enough.

So if you Goal is to play the most DOS games with good performance then get a Pentium CPU.
Also the 486 can NOT play MP3’s.

I'm not really looking for a 486 to play games with - looking for a 486 to make old DOS games with.

Then you probably want to go with the lowest speed 486 CPU which is the 486dx-33 to support the largest 486 CPU group.
The 486dx2-66 was the best value and very popular 486 of its time.
The 486dx4-100 offers only about 10% CPU boost over the 486dx2-66

Is 150W enough for a 486 computer? (And should I be worried about a PSU having 0.3A on the -5V rail?)

150W is more than enough. I don't know enough to comment on the -5V though.

Does anybody have any insight on the whole -5V rail @ 0.3A thing?

Negative 5V is usually irrelevant in terms of capacity, while it is required for some older add-on boards. Some older sound cards and what-not require it.
But in terms of current, it's pretty irrelevant.