I stumbled into this in a Google search, and though its an old thread I'd still like to respond in case you or anyone else is still concerned. I'm disappointed to read Tyan's response they gave you, I've never dealt with their tech support but they don't sound like they try very hard.
I have the same motherboard, it was a current model in late 1998 and definitely supports Win98, even though they didn't acknowledge that. VIA also has driver support for that chipset under Windows 2000, I've installed that a few times and am setting it up again right now. I'm not sure what they mean about ACPI, because the bios clearly has ACPI settings in it, but I've never used them since I have it on an AT power supply. The problem with this board is its from the early days of AGP, when non-Intel chipsets didn't handle AGP very well, so you have to fight the chipset to make it work.
Power (watts) might also be an issue. I remember reading articles on the internet indicating that many early AGP motherboards were getting blown out by TNT2 video cards, because the early boards weren't able to handle the high amperage that the newer cards were trying to pull. This happened to a friend of mine on a different motherboard which is what caused me to find out about it. I don't know if the Tyan S1590 has this problem or not, but I didn't want to find out. I got a GeForce2 MX, which is really a perfect match since its fanless, low-power, and is fast enough to saturate the CPU in 3D. I had trouble getting it to work but it will work. You have a "400", which is newer than the one I had but I doubt there's much difference on compatibility. I also have installed a much newer ATI AIW-9000 on a different AGP 2X machine, so I expect there probably isn't anything incompatible about that card.
Its been a long time since I dealt with this so unfortunately I don't remember the whole procedure, but there were a few things I remember being key.
1. Enable USB in the bios, even if you aren't using it. For some reason, AGP will not work without this turned on. Its some weird quirk of the chipset. My system used to boot to a black screen or standard VGA without this.
2. Install the VIA agp drivers in "normal" mode, not turbo. That will force it to run in AGP 1X, instead of 2X. AGP 2X seemed to be unstable for me, and even when it worked, I tried benchmarking in 3DMark and found that there was virtually no difference at all in the performance, probably because of the CPU limitation.
Trying to do this in 2000, I can't get it to prompt me for "normal" mode. I read a comment on another site though that implied it always uses "normal" mode when installing these drivers on 2000. Whatever it does on 2000 is probably the same on XP. There's supposed to be a registry key at
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\VxD\VIAGART
that enables turbo mode if its present. The "VxD\" registry folder doesn't exist on my system that I'm installing right now, so I'm guessing its in normal mode. This is with the 4-in-1 v4.43 drivers. I don't have an AGP card on the board anymore so I have no way to tell if its really working or not.
I think I may have had to use an older version of the VIA drivers, but I don't remember which it was and I'm not sure that was really an issue. Whatever version I was using back then was probably 4.28 or so. I do remember that the old nvidia 5.32 drivers were faster than everything newer for some reason, so I used them.
There's an option in the BIOS to enable "1 WS AGP writes" or something like that. I think it's enabled by default but if yours isn't then it probably should be. My BIOS back when I had AGP working was v1.16. I have no reason to think that's an issue though.
Another option - there are PCI cards with decent 3D on them. I remember specifically that there was an eVGA PCI card with the same GeForce2 MX on it, which I considered ordering until I figured out the AGP problem. If you're still trying to work this out and want some 3D, you might just look for a PCI version on ebay. Performance wise, I remember that the 3dMark scores back in the day showed that a TNT2 did not saturate a K6-3 processor - the GF2 MX had better scores. Not sure what CPU you're using but I'd try not to go below the MX or you'll end up with less 3D performance.
