The frame rate was ok on my nvidia 260GTX... mind you I forced Antiliasing and Aniso to the max in the nvidia control panel for that game and it was decent framerate 30+fps thru out (on 1.00)
Running the game on a GTX275 myself which is pretty much the same card and I'm having horrible framerates without AA/AF and while playing on default settings at 1920x1200 :/
My system doesn't appear to have any bottlenecks that may affect the game. The game stutters while two out of four CPU cores are at about 60-80% and the GPU at 50% load. (i5-750 @ 3.6GHz, 8GB DDR3-RAM, Win7 x64) Changing affinity to one core doesn't help at all but puts the core in use to 100% load.
So the problem doesn't seem to be related to the card model/manufacturer only.