It depends on the Mac, however. The G5 Powermacs have a AGP Proish type of connector that looks like it needs a special card.
Here's a pic of one of the Special Edition 9800 Pros for the G5 that have DVI+ADC connectors. You might be able to get a regular card to fit into a G5, but I'm not sure how it will work.
Of course, next year it is possible EVERY PCIe graohics card will be compatable if Apple uses a standard x16 interface on their new Intel Mac mobos. It seems needlessly complaicated and expensive to do otherwise, but you never know with Apple.