A geforce 6800 GT (or a 7800) which i think is a bit more powerful then the 9800 pro, even in a G4 this would be enough to drive most games. let alone a dual 1.4ghz or 1.8ghz G4.
there is a site
www.themacelite.com the allows you to put a way more powerful video card in a Mac then a 9800 pro (if its a G4) which is not a bad card by any means, but a geforce 6800 GT (or 7800) will stomp one easily. a G5 will benefit even more from one. but the Makers/porting houses are looking at what the system came shipped with and its BTO options and not what it can be upgraded to.