I am running the game on Ultra settings on 1680x1050 with no issues.
What fixed the framerate for me was to go under 3D Application Settings under Gaming in ATI Catalyst Settings and remove the checkmark on Use Application Settings under Smoothvision HD:Anti-Aliasing, leave the level at 2X and do not enable Morphological Filtering. Under game settings disable Vsync and adjust the Buffer Frames to 1. I am running the game on Ultra settings, only shadows on Very High with steady framerate and no issues.
I am running the game smooth as a butter on AMD Phenom II 955, 8GB DDR3, ATI Radeon HD 6870 1GB GDDR5, Win7 Ultimate 64bit.
I hope this helps. ;)