I have that one and play Diablo in full hd with decent settings in OS X. It performs well with that game. I have also tried CS Source and that plays well. You will have to tweak the settings some if you want to play in native resolution. Lower shadows and such to get the frame rate up.
I would look at benchmarks if gaming is important. The future iMac will Have a better gpu though, probably the same as the new MacBook pro, gt 650m.