Not that much...
nVidia only has two options, the stock FX5200 or the 6800.
ATI has a bit more:
Radeon 9600 PowerMac G5 edition or the PC&Mac edition, though it is not much more powerfull than the stock nVidia 5200.
Radeon 9800 Pro Mac Edition and Special Edition. The difference is that the special edition has 256MB (128MB for the other) and AGP 8x (AGP 4x).
Radeon X800 XT Mac edition.
I have a 9800 Mac edition in my G5, which was previously installed in my MDD. It's quite a powerfull card, and more than enough for my needs. I don't do heavy gaming on my Mac though, that's why the Xbox is for.