FYI
My system:
AMD64 3800+
2Gb RAM
2 x 7800GTX in SLi
Settings:
1920 x 1200 resolution
Visibility: 1200m
Terrain details: Normal
Object details: Low
Texture details: Very Low
Shading details: Normal
Postprocess effects: Low
Anisotropic Filtering: Very High
Shadow detail: Disabled
Antialiasing: Disabled
Blood: High
Performance (fraps):
Average 50fps, in build up areas and forest: 25-40fps
Comments:
I have found that switching of shadows and antialiasing and keeping texture details very low, you can boost performance a lot.