Well, it has been over half a year that I sold my iMac so I can't give you the precise settings... are you sure you have the high-end version (with the 6970M)? Obviously you should disable anti-aliasing. Also, you can play around with shadow settings. I am rather sure though that I left everything on max (with AA disabled). Most benchmarks show AMD 6850 (basically the same card, slightly faster) getting around 70 fps on high settings in Skyrim at 1920x1200 with 4xAA.
Yeah, I have the 6970M with 1 GB ram on the card.
I have tried different drivers as well, but it could be Windows 8 or the installation of Skyrim itself that is messing things up. OK, thanks for you help anyway. I actually had to put everything on low and set the resolution really low (720p) to be able to play.