If you plan on keeping your Mac for say at least 3 years buy the fastest card you can afford.
Panther already leans heavily on the GPU for the Quartz Extreme display system. Tiger, coming out by mid 2005, will rely even more on the GPU. Apple is taking so much of the windowing core tech and pushing that off to the GPU to free up your CPU. Thus..the faster your GPU is the faster your OS and applications will run.
While I'd hesitate to recommend the new nvidia 6800 for you because of its $599 cost(less whatever the standard G5 card is BTO) you may want to get the fastest ATI card 9800XT for an additional $350. Going forward fast GPUs are going to be far more useful than just gaming.