For what it's worth I play World of Warcraft it is quite graphics intensive for quite a few hours on end. I recently got the quad core 27" iMac.
I love it, no hitches, or glitches... well actually. Only twice in at least 60+hours of gaming have I noticed that the terrain wasnt rendering correctly, I logged out and then back in and all was well.
It runs freaking hot towards the top so I have a small and silent fan behind the unit and it keeps the temperatures down quite nicely. Monitor is so large that I don't even see it or really feel it lol!
Will post new info if anything changes.
Cheers!