If you plan on gaming with your Mac, good video cards are a hefty investment. If I were you, I'd get by with the 9600 for now, and save longer for a better card. The 9600 isn't a slouch, but it's not exactly the best either.
I'd wait a few months to see if the new Nvidia 7800 cards become available for Mac, as these babies are virtually indestructible on the PC side of things. They probably won't ever make an AGP version for G5s though, so you may want to go ahead and spring for an ATI X800XT. To me, spending over $200 on a 9800 SE when you can get much better performance for $400 - $500 is plain crazy.
Another option you have is to buy the PC version of the Nvidia 6800GT and flash it to work on a Mac, although I'm not sure if all the features will still work.