I see that the new iMac's are getting a special video card from nVidia that apparently is only available to them... at least currently. The specs on this thing look pretty awesome (more CUDA cores, 2500MHz memory clock and it will do 4K2K monitors):
NVIDIA GeForce GTX 680MX - Notebookcheck.net Tech
Too bad you can't get this sucker in a gaming laptop... it would kick ass!!! They are saying it will perform similarly to a GTX580.