I have to agree that the graphics processor will be your limiting factor in games in the future, not the CPU. The GPU in the aluminum iMac seems to have been designed without games in mind; while it works well for some recent games at standard settings, anti-aliasing completely removes usability. Even on games where the anti-aliasing doesn't lower performance below usability standards there is still a dropped frame every second or two, which makes all games appear as if they are stuttering.
The GPU is my biggest complaint. I know that it's too hard to expect a fantastic GPU in what is basically a mid-range laptop in a desktop form-factor, but they did put a pretty darn good GPU in the MacBook Pros...