Looks like you only have space for single slot graphics cards. That will limit you a bit. The voodoos can handle older games so I think backwards compatibility of the new card is less of a concern.

Radeon 9700 Pro and 9800 pro are good for early XP games but are expensive and prone to die. Midrange 9000 series are cheaper and less prone to die. X800 series are even more powerful, but the better cards (like x850 PE) are dual slot and expensive.

I think I would avoid nVidia as FX series is poor at directx 9,and 6000 series has bad drivers for win 98.

I have several X800 agp cards, but they seem as an overkill for it, and I have a small shuttle xpc s939 agp with that card, so don't know to pursue that path again. Let'sa say I want to play games which cannot be played on Win10, that's my limit.

I have a MSI FX5900VTD-256 with , are they prone to die?

Sounds like you don't really need this machine for XP or directx 9 stuff if you have that other machine?

I think FX5900 is great for Win98, but I dont think you need directX 9 support for that OS really, as those games are more XP era.

Or are you thinking of using this machine for all Win98 / XP gaming (it will definitely struggle with later XP games whatever GPU you use).

Yes, my plan is to have dual boot 98 and Xp, and everything kater I can run on newer setup with Win10 to run there (i7 4th gen and rx590).

Shuttle XPC is tucked away as I don't have enough space on my working bench, and it's also a dual core CPU (experimenting with athlin64 x2 on nforce3 chipset).

If that is your goal I would say put in the most powerful GPU you have (x800 maybe), but you will find the CPU is a bottleneck for later XP games.

If it were me, I think I would leave the Win9x machine as just for 9x (possibly upgrading GPU to FX 5900) and then dual boot the 4th gen i7 with XP and Windows 10 and swap the RX590 for a GTX 980 Ti 6gb. Less VRAM, but I think slightly faster, and is compatible with XP (with modded drivers).

So in your opinion what would be the appropriate (maxed out) machine for Win98se, and which graphics card.. Maybe I should downgrade 😁

I think your FX5900 would be great for a maxed out Win98 build.

My maxed out Win98 build has core 2 Duo X6800 and Geforce FX 5900. Most games run great, but there are a few problematic ones. I think Viper Racing, Klingon Honor Gurard and Heavy Gear didn't work. I forget what others, but I probably have close to 100 games on it that do work.