For what it is worth OP you might want to upgrade your CPU as well if possible. I was running an Athlon X2 @3.0GHz with 4GB RAM and an nVidia GTX 470. The game was very playable (20-30 FPS most of the time) but I later upgraded to a Core i7 @3.3GHz and 6GB RAM. Using the exact same video card I'm getting anywhere from 45-60FPS depending on where I am and how many people are around. Seemed like my old CPU was a big bottleneck and holding the video card back (not just on this game, but others as well).

Settings I use are 4xMSAA, high settings for most everything, DoF and Hardware Mouse ON, AO off. Resolution is at 1920x1200 in windowed mode. Running on Windows 7 Home Premium x64.