ATI Radeon 5870 1GB
AMD Phenom II X4 Black Edition 3.2GHz
4GB RAM
1920x1080
Windows 7 64-bit

Tried setting the shadows to low, barely any difference.
Running on Medium, still pretty laggy.
iPresentInterval=0, removed V-Sync but still lags.
Barely any other background processes.
Every other game runs perfectly apart from this one.

Anything else I could try? =/