If it's the one I am thinking of it's a radeon 9000. Otheriwse it the Nvidia Geforce 4.
@commodorejohn
nah he is talking about the g4's. Again lots of after market graphics cards but the two that came from Apple were Nvidia geforce or Radeon 9000's.
edit==
Argh looking fruther it appears the radeon 9000 was the standard card and the geforce was the upgrade card. If you have the system it should say in the about this mac box on the OS.