I didn't think you could still buy the 9600, they're that old.

www.tomshardware.com - PC centric, but take a look at the VGA charts to get an idea of what sort of card you should be looking at. At a guess I'd say that spending about $200-$300 USD will get you what you need. Not sure of the mac compatability of the Nvidia 6600GT, but they're a good midrange card, as is the upcoming Nvidia 7800GS.

Actually, I'd just recommend that you save up pennies and buy a seperate gaming PC.