Sweet. I would suggest cranking shader as high as possible, and holding off on bringing shadows, particles, volumetric effects, post processing and game effects above medium unless you have lots of frames to spare. Putting these on medium still has the game looking 90% as good as all on high, but with alot of extra frames.
As an example, i have a single 2900pro at stock speeds and a core 2 duo @2.6ghz with windows xp, I use these settings (see below) and get 35-40 fps during the vast majority of the game (at least everything until just after blowing up the petrol station and rescuing the hostage, as I havent played further than that yet). Its smooth and looks gorgeous. Havent tried the patch yet either (no time!). Changing to 1400x900 looks alot sharper with the extra resolution, but I come down to about 27-32 fps, staying in that narrow band most of the time. A single 3870 should handle this resolutions at these settings very well.
Resolution: 1152x864
AA: off
Shader: High
Object detail: High
Texture quality: High
Physics quality: High
Shadow quality: Medium
Volumetric effects quality: Medium
Game effects quality: Medium
Postprocessing quality: Medium
Particles quality: Medium
Water quality: High
Sound Quality: High