also I recommend getting the 3850 agp model, although a fresh computer would be the best choice. My computer is worse than yours yet I experience similar problems.
Pentium 4 1.6GHz
512 RAM
GeForce 2 MX200 AGP 4x
Highest - 50 fps, aerial view, singleplayer
Average - 15 fps, third person
Grid - 5 fps (1 fps if in cockpit)