Sorry if this has been covered already.
If you are running the x64 executable then change the shortcut to 'bin32' instead and get a nice boost. The x64 version is slower than the 32 bit version for some dumb reason. In x64 XP anyway. This was verified by running the x32 and x64 GPU/CPU benchmarks with the same settings.
I went from unplayable to 1440x900 with Very High settings, 8x CSAA and 8x AF with my lowly system listed below:
XP Pro x64
8800 GTS 640MB stock
AMD X2 4200+
2 GB DDR
With a Quad core upgrade this game will be very playable (in DX9 anyway)
I am not aware of any extra content in the x64 version (like Far Cry) so they definately have some headroom to optimize the x64 version.