Why do you say the S754 isn't fully backwards compatible? No options for ISA or PC/PCI support (true)?

Yes, that is correct.
Also the boards with nforce4-4x chipset does not natively support windows 9X and i plan to have 2 different hdd, one wtih windows 98se, one with windows XP.
But the main reason is in my country it is very difficult to find athlon64 for socket 754 but very easy to find sempron (also very cheap, something like 10 euro or less).

Well, as far as driver longevity, I think your best option is to go Taiwanese and buy a VIA board. The only loss you suffer is SLI which I think should be used with a newer XP/7 build anyway.

On what socket? I had a via motherboard with kt400 chipset and it was awful regarding performance; a friend with nforce2 had more fps in every game with the same CPU and VGA, if we switched boards I would get "his fps" and he would get "mine fps" despite both having duron and geforce4 (mx i think).

KT600 or KT880 were competitive with NVidia, but there's no problem with NForce2 and Win98. I was thinking of 754, 939, and AM2.

1995 - 2005 is very hard to achieve.

1995 - 2002 should be the Win98 <1GHz domain.
You could run into problems with the PC being either to fast for DOS games or being to slow for DX7, 8 or 8.1 games.
I would not use more than 256 or max 512MB RAM.
A good AGP graphics card with full Direct 8.1 features (Pixel and Vertex shaders) card should play all the games with a smooth framerate.
To round it off add an ISA soundcard to the build with good DOS compatibility and no collectors value.
AWE 64 value and Sound Blaster 32 should be good choices or a Soundblaster 128 at the least.

2002 - 2005 I would go for an Athlon 64 (or X2) or Pentium D CPU. They should be very easy and cheap to get. You can use all of your DDR1 sticks and have the choice between AGP and PCIe cards. This should be the cheaper system.

This is just the minimum I would go for, you can of course use more delicate parts if you want

I would go a little bit further, and say "A w98 rig for 1995-2002 is what you need and very achievable (I also suggest PIII Tualatin), but 2002+ and XP is not good in the same machine, plus you DON'T even need it."

I'd start with deciding on a motherboard/CPU then select go form there.

Personally I recommend the below keeping in mind your main criteria was:
Win98 to WinXP (no Dos focus)
Music and office will run on even older hardware, movies maybe not but you should be ok if its not doing anything else and your not trying to stream stuff of the web.

Socket A, if your dream computer was this then get it!
Double check you can get Win98 drivers for the motherboard you find, as long as you stay with the main guys like Gigiabyte, Asus, etc you should be safe.

If you are after pure speed, I would go for socket 478 they are cheap, 865 or 875 chipsets still have good native Win98 support, may struggle with XP games?

Not familiar enough with Socket 754 or 939 to say one way or another.

I would also stick with AGP, you can hack 98 to work with PCI Express cards but more trouble then its worth IMHO.
Finally for your hard drives, Just get a IDE to SATA converter off ebay, something like the below
http://www.ebay.co.uk/itm/New-3-5-PATA-IDE-to … n0AAOxyHt9R3oPZ

But really, if you wanted a Barton enough to mention it above, I think that's the right choice for you

I eliminated socket 939 from the list because chipset compatibility with windows 9x is sketchy at best after reading over 20 motherboards manuals! Also PCI express GPU do not play well with windows 9X. Also I drop windows XP compatibility because I have a core duo for that.
What I would like to know is the answer to question 2: do I need to worry about the power supply or no? Would one of the aforementioned PSU will work in a socket 478/A/754 motherboard with a 20 pin ATX connector?
PS: back in 2003-2004 I was dreaming about abit nf7-s board but I couldn't afford it and now they are all gone, I couldn't find one despite watching the local market from over a month.
PS2: the system will not be connected to the internet, just to my in house network.

The main issue with using newer powersupplies on older systems is the shift of focus from 5v/3.3v to 12v. Usually this is not an issue if you are using 478, since 478 is 12v heavy. If you are going for Socket A pick whichever powersupply has around 25+ amps or so on the 3.3 and 5v rails.

I tend to ignore motherboard manuals and pay attention to driver documentation when it comes to software support 😀

If the motherboard you go with has a 4 or 8 pin 12V connector, it's almost guaranteed to drive the CPU using VRMs from the 12V rail. That would make it compatible with any modern PSU and you won't need a particularly strong 5V rail.

After careful considerations I consider the 2 remaining choices to be: scoket 478 and socket A; both have full windows 9X/2000 support, have AGP video cards and I don't have to use any "modded" software. I rejected socket 754 because a friend of mine also with old games nostalgia tried and failed to install windows 98 se on an asus k8n4e deluxe motherboard (even after heavily modifying windows he could not succeed in getting the system stable).
Now I found some CPU and motherboards on the local equivalent of ebay and I wonder wich one would be a good choice:
1. pentium 4 prescott + intel 865 chipset motherboards: from asus, gigabyte and msi.
2. athlon xp barton + nforce2 ultra/via kt880 chipset motherboard: from asus, abit, gigabyte.
Should I go the pentium4 or the athlon xp route?

nd22 wrote:

....I rejected socket 754 because a friend of mine also with old games nostalgia tried and failed to install windows 98 se on an asus k8n4e deluxe motherboard....

One of the boards that I tried and failed with official/included Windows 98 drivers was an Asus K8N-E Deluxe. 🤣

I would suggest this:

1. pentium 4 prescott + intel 865 chipset motherboards: from asus, gigabyte and msi.

Availability and cheapness is the deciding factor here, for me at least. And does not generally need a 5V heavy PSU of the past, like most socket A setups does.

To say the truth, this is just the answer to your question about P4 vs. Athlon. I personally would go for a Pentium III setup. Not that cheap/widely available but more fun in my opinion. 😊

On the local equivalent of eBay an Abit AN7 just appeared; the seller still has the I/O shield and all the cables and say s is a fully functional board minus the north bridge fan that makes lots of noise! This motherdoard has a 4 pin pentium 4 style CPU connector! Also he is willing to give me a duron of unknown frequency and a stock AMD cooler. I think I am going to take chinny22 advice and build a socket A dream computer!
My only question is: with this board should I still worry about the PSU or not?

The manual still specifies a strong 5V rail, so I would source a period power supply for this build. Try to get something solid and reputable like a Delta.

This is a picture of corsair TX 750w v2 PSU that I have; it is 25A on 3.3V and 5V:
Shoud this be enough or should I search for a Delta PSU?
PS: also found an athlon xp 3200 with fsb400 which I know it is the most powerful socket A chip unless I could find the extremely rare Athlon xp 3200 with fsb 333.

Nope, you need at least 30A on the 5V rail for an XP 3200. I recommend a Delta PSU because they were commonly available in OEM systems (so there's availability), parts selection and construction is excellent, and the design is very robust with the stated capabilities being very conservative. Furthermore, load tables are available for pretty much every model PSU out there.

Look for a model in the DPS-300, DPS-350, or DPS-400 families. Some of the later sub types are ATX12V for use on Pentium IV rigs, but many are ATX 1.3 with a heavy 5V rail. Here's one on ePay:
http://www.ebay.com/itm/Delta-DPS-350NB-1-20- … MMAAOSwmLlYB5a6

The nice thing about Delta power supplies is that if they say they can give you 30A, you'll get 30A solid, day in, day out. Similar with manufacturers like Zippy, Liteon, and Etasis, You'll see all these brands listed as OEM parts from Dell, HP, IBM, Apple, etc. I've included some other units which would be suitable:
http://www.ebay.com/itm/EMACS-ZIPPY-HG2-6300P … rYAAOSwIgNXtHRo
http://www.ebay.com/itm/Zippy-Emacs-400W-ATX- … BwAAOxySoJTV9tc

One caveat with supplies from Dell computers is that a handful of models used ATX connectors with non-standard pinouts. Definitely avoid those types.
http://www.overclockers.com/forums/archive/in … p/t-211595.html

Didn't say your dream PC will be easy 😉
Although you already found the board and CPU, rekon that's the hardest part done. If you end up struggling you may find another PC from that era you can get just to salvage the power supply.
Doubt you'll regret it though. I keep going back to my "dream" 486 even though I have other systems that play games better but playing on that gives me the biggest warm and fuzzy feeling over the others

There are S775 Systems with ISA.

PICMIG Industrial Boards, even with Core Quad 😀