Looking over some video's from the previous iMac and it ran ok (Arma2)
http://www.youtube.com/watch?v=ntSFofFd6GI
Is the GPU on the new iMac superior or inferior ?
I'm not sure if I would call that ok, though I'm sure the screen recording took a toll. Frames are dropping and the screen is tearing at every turn.
In any case, I went through and selected the model you saw in the video (at the bottom due to it being the weakest), and the models currently available with the new iMac's.
To put it simply, yes they are all more powerful, however the starting GPU is only a little more powerful than that ATI (though given the 640 is nvidia, the drivers are probably a lot better...).
Then lastly, the 680MX is more or less the standard for mobile GPU's.
For you however, with the 2.9 model you'll be using the 660M, which according to that same site will get you ~37FPS at high settings for BF3. So it looks to be pretty great for it there. Ultra however, is out of the question.
Lastly, there were no Arma 2 benchmarks...