The GPU on the high end 27" is pretty damn close to one. You'll easily be able to run just about any game at native resolution with all the settings bumped up about as high as they'll go.
Don't count on it, the top end games like BF3 is going to struggle at max on that 2560x1440 display. Even the MX.
The MX will be about 10-15% faster than the regular well benchmarked 680M in BF3 at 1080p you're looking at a 33FPS average, so if we're being generous and we see a 15% increase in FPS it's about 38FPS on 1080p. Pushing about 25% more pixels than 1080p you're looking at about 20-25FPS.
Turn down some settings and it might reach 60 but it's not going to be near max, I'd say medium at best, and I'm not too sure about the CPU as it's a fairly CPU taxing game.
I bet the MX options also going to cost a fortune, and here's the best part, in 1.5-2.0 years when games are pushing hardware forward you've got no upgrade avenue besides to turn down settings and down.
Not trying to be a dick but if you're even semi serious about gaming on a PC you might as well just build your own and get a cheaper iMac, by the time you pay for all the upgrades it's probably going to be a cheaper option than souping up an iMac that isn't going to be able to cut the mustard now and into the future.
no mention of the retarded location for the SD card slot.
Bu... bu... but it's thin!