I just upgraded my Dual G4 video card. I upgraded it from the factory Rage Pro AGP (16MB), to the Radeon 9000 Pro (128MB) AGP. From a bunch of reading i found out that this is the most up to date Radeon card that will work with the Gigabit G4's. It is the only Radeon card that is 4x/2x compatable. If you were thinking about getting the Radeon 9200 PCI, i'm not sure if it would be as fast as the AGP card even though it is newer than the 9000.
ATI Radeon 9000
here is an eBay auction, i bought mine from these guys:
geekdeal: ATI Radeon 9000 128MB AGP
hope this helps...