I have the 27" with the 3.2 i5 and the 680MX and it honestly doesn't struggle with any games right now.
Most of the standard stuff released in a last couple of years (and obviously earlier than that) can be run at max settings at 2560x1440 without a problem.
For example, I can crank Skyrim right up, and add in the extras like Ambient Occlusion and high red textures and the iMac doesn't even break sweat. Looks outstanding, plays beautifully. (This is in bootcamp on Win7 64).
I can play Planetside 2 at "High" settings at 2560x1440 and I get between 20-50 fps (the low end is due to CPU bottleneck, the high end is GPU bottleneck). Some people will clearly not be happy with sub-60 fps for a first person shooter, but for those people you could just drop it down to 1080p and get a big frame boost, but I haven't found it to be strictly necessary. If you want to play this, your i7 is going to help you more than my i5 does - the low FPS drops are entirely down to the CPU.
Xcom: Enemy Unknown - Put it on absolute max settings and laugh. Looks lovely like this, but it is hardly taxing on the GPU.
Fallout: New Vegas - again, put it all on the best settings and enjoy.
Now, the big crunch will probably be something like Metro 2033 (or even heavier, Metro: Last Light). I own the former but have yet to play it, but I suspect the legendary GPU taxer is going to pose much more of a challenge. I'll probably be playing it at 1080p.
If you want a comparison of a more common GPU to make comparisons to, go with the desktop 660 Ti - it is about the same in performance as the 680MX, so if you use that as your baseline you'll get a feel for how particular games will run on your system (although remember, most benchmarks are done at 1080p).