I've got the i7 iMac. In Boot Camp (Windows 7 Pro, 64-bit), I've run the following games so far:
L4D2 demo
Crysis Warhead
STALKER: Shadow of Chernobyl
L4D2's recommended settings set me up at 1900x1200 with all game settings on High and 4xMSAA. I bumped it up to the screen's native res and took off MSAA. It looks gorgeous game proved to be very playable. I turned on the console FPS graph (cl_showfps 1 in console) and the frames were typically between 30 and 60 (had vsync enabled, double buffering), at times it dipped below 30. So like I said, this is totally playable, but I did notice some stuttering that affected my aim during gameplay (was using a USB PC mouse, not Magic Mouse). I found two ways to get rid of the stuttering: either bump down the shader detail to Medium (boooo) or change the res to 1080x720 and bump the MSAA back up to 4x. The latter method was my preferred choice because the shader details give the game it's character.
I didn't get to play as much Crysis Warhead, but the game recommended that my settings all be set to Gamer (which is High). I put the resolution at 1080x720 because there's no way the game will run well at native res. I didn't get into any particularly huge firefights, but the game runs at a respectable rate (I'd guess >30 FPS, though I didn't set a console graph or use FRAPS to measure it).
Same thing for STALKER, except that ran on High at native res with similar frame rates. Downloaded the STALKER Complete Mod 2009 (which overhauls the graphics), but didn't get the chance to install it yet, so I'm not sure how that will affect the frame rate. Vanilla STALKER ran great, though (except for a no sound issue I'm having--buggy game).
I'll be trying Overlord 2, TF2 and a few others this weekend.
