Halo and Doom3 are about the only big name first person shooters to come out recently for the Mac. You can get America's Army as well, and maybe Unreal Tournament 2004, but after that the choices quickly drop. All of Blizzard's games are available on the Mac, so you do have some choice. Ambrosia Software makes some great Mac only games though.
The G5 processor is great for games, if companies would take advantage of it. A larger problem is that Apple put a horribly outdated video card in the new iMac, and a somewhat less outdated video card in the towers. The choices for better video cards isn't exactly overwhelming either.
If you can afford a G5 Tower, I bet you could afford a XBox or PS2 as well. Use the G5 for work, and the console for gaming. I have many friend's who have pretty much given up their PCs for a console anyways.
goates