I recently replaced my Prophet 2 MX 32MB with an ATI Radeon 9550 256MB (it was < £50) and increased my FPS from 15-20 to between 40 & 60 depending upon how many other cars are running.
My rig is AMD XP2000 512MB DDR not o/c.
Apparently, the 9550 is pretty good for overclocking, but I haven't felt the need.