"Despite its new ATI Radeon HD 2600 Pro graphics chip, the iMac still isn't very well suited to playing 3D games. On our Quake 4 test, at a forgiving 1,024x768 resolution, the iMac turned in an only marginally acceptable 39 frames per second. We were surprised by that, since Quake 4 sits on the tail end of what we consider current 3D games. Comparable Windows PCs from Dell and Velocity Micro perform much better. You should be able to play less-advanced 3D games on the iMac, but we're still disappointed that Apple doesn't want to take gaming seriously."
http://reviews.cnet.com/desktops/apple-imac-20-inch/4505-3118_7-32564364.html?tag=bubbl_2
http://reviews.cnet.com/desktops/apple-imac-20-inch/4505-3118_7-32564364.html?tag=bubbl_2