I've heard that even on high end systems it can go slow and jitter. Apparently the game makers were in a rush to get it out that the programming wasn't the best, hence causing jitter problems in the game. I wouldn't bother upgrading ya comp.
You might want to try turning off the trails feature and the frame sync feature. If you still have problems lower the draw distance. After I adjusted those settings everything worked fine. My specs are:
1.4ghz (thunderbird) Athlon (266)
ATI Radeon 64DDR VIVO