Try maxing your resolution to what your computer can handle, if 1600x1200 is possible, go for it. Your card will work best then.
I have a 800mhz and a GeForce 4 Ti4200 64mb at 1280x1024, pretty much all but heat haze and dynamic enviro mapping in the forward view, just cars in the rear, and I get a pretty steady 24 FPS (calibrates to 22).