I've had a few games tonight on my 3.1, 12GB ram, 1GB 6970m, 27" iMac and it played very smoothly at the auto settings, which was high on everything apart from AA (med) and at native resolution. Looked really nice as well. I've heard that the Caspian map looks a LOT better.
To be honest, I didn't think it would run it that well. Definitely enough to get me to preorder. There's talk of the settings not being final though as ultra is no better than high.