pIllusion uses basic OpenGL features, so you don't need an advanced GPU, but faster is always better. If you look at game benchmarks, that's a pretty good measure of how it will work in pIllusion.
More memory on the card is always better too: I'd suggest at least 128 MB, but 256 would be better.
I haven't used ATI cards except in my Macs, so I can't report on how they work (although there are some pretty basic OpenGL features that ATI OS X drivers don't support -- like drawing antialiased lines). I've used NVidia-based cards in all of my PCs since my original TNT.