I got some some lag running at 1680x1050, with the graphics on max except for AA set to 2x. So I switched AA off and turned AF down to 8x and I'm not getting any lag at all now. My specs are:
Windows Vista
AMD Athlon 4600+ (running at 2.3 GHz)
1.5 GB DDR RAM.
ATI Radeon HD 3850 256MB
Running at 1680x1050
Im not very far into the game but it does feel laggy even though my framerate is high. for instance when I turn with the flashlight there is a lot of mouse movement delay that wasnt there in the other two games.