Hey so currently im running a 1100T and a pair of 6950's with the latest drivers etc..

However atm im finding my frame rates terribly low im not sure if its my Gpu's that are faulty or if this kind of performance is to be expected. Please note i have had lots of issues with X-fire with prior games to this.

Anyway atm im running everything on:

High (water and shadows turned to medium)
MSAA x 2
antistropic filtering - 4x (played around with it didnt do much)
motion blur off
V sync off

During the first mission in the start im getting
18-20 FPS @ 1920x1080 or 11 FPS @ 2560x1440

after the rain whilst im in the corridors i get:
24-35 @ 1920x1080 with alot of stutter game is basically unplayable when i need to run and shoot and not stealth killing everything while standing still

or ~19 FPS @ 2560x1440

The first mission has an optimization issue with the moving ropes. You should get better framerate after that. Also, for me SMAA runs a lot smoother than MSAA without much visible difference in graphics.