If you are serious about gaming, I say skip this generation and the next generation altogether and build your own gaming rig.
I have the current i7 model and for all intents and purposes, it is a beast and chews through anything I throw at it, with the exception of games at native resolution.
I currently play COD WAW and Black Ops, Medal of Honor and Left 4 Dead 2. I run full details/high quality at 1920 x 1050 (or whatever that 2nd number is) and get online FPS at 50 and above in heavy situations, which is more than respectable but if you are REALLY into gaming, you probably want higher resolutions and maybe even better framerates.
If I run the same games at the same detail settings and native resolution (2560 x 1440) FPS is 30 to 40's and IMO that is not a good framerate for online FPS. Single Player is different, it looks amazing and I'll play that way all day long.
Waiting on the next gen will probably still be a disappointment for the gaming world as we are only going to see a 10-15% performance boost over the current i7. Sandy Bridge and a 6850 are not going to give the current i7 the boost that the people in the macbook world have seen compared to prior generations
To close, if the FPS numbers I mentioned above are acceptable to you, by all means get the iMac. I know I do not regret it nor ever think about a more powerful rig. I would suggest waiting though, the new hardware should be out soon and you might as well get every FPS with the money you spend.