Haven't had a problem with my rig at all. Smooth frame rates, all the time. This is with 4xAA at 1920x1200 too.
I mean, a new CPU might do me good, but I'm trying to hold off till 2011 on that. This little Q6600 has served me quite well.

Edit: Forgot to say, I'm on 10.7a drivers. Though that should be obvious with the "4xAA" part.