If you are a serious gamer
do not buy a Mac. It's as simple as that. The new nVidia GeForce 8800 range for desktops smokes any card available for Mac right now.
The X1600 found in iMac and MacBook Pro will play older games fine but when it comes to getting recent games with lots of new visual effects to work you'll see a significant drop in performance and framerates.
That said I can't think of a good recent PC game I'd like to play. In my opinion, an Xbox 360 + Mac is the best setup to have these days