G5 Cards will not work either because of the pin configuration (they're usually agp pro). You can flash a PC card which isn't reliable, I tried and had no luck with several cards. Check out Strangedogs.com. Or track down an older nvidia card, Ati 9000, 9200 or 9800 (2x/4x) for mac. Unfortunately only the 9800 has core image compatibility.
I deem the G4 sawtooth as not a gaming machine unless you are playing games from pre-2002. Or you don't mind slow FPS, which I couldn't tolerate.