The GPU in the iMac is not really faster than the 6850 desktop version.
If anything, it's slower, but by like 5-10%.
Playing WoW in 10.6.8, I get 60fps all the time. Haven't been in a 25 man raid in a while (maybe when Firelands hits next week,) but I'm happy with the GPU performance.
I turn shadows to lowest, liquid detail to lowest, and sunshafts are disabled. 2x anti-aliasing and 1x multi-sampling for me (rather have the guaranteed 60fps with V-sync on, eye candy isn't as important than running at native res, any other res looks fugly on this screen.) Added benefit of keeping the graphics fairly low (for this machine,) is that the fan doesn't come on for the GPU. If I turn my vsync off and let the FPS go to max, I get 200fps, and then the fans start coming on.
On Win7, I can turn up the graphics and still stay at 60fps, but the GPU does heat up and my keybinds don't translate over that nicely from OS X, so I play in OS X.
And yes, the SSD is huge for zoning in, hearthing to a populated city, etc. WoW from the character screen to play is like 4 seconds max for me. Scrambling to reconnect after a disconnect is even quicker. Takes longer for me to get my authenticator code, press the button on it, and enter it.