Hard drive did make a difference to this game. If you can, install the game on SSD. My situation is that I have to defrag the hard drive in order to eliminate the stutter that is not graphic card and CPU related.
In your situation, I would recommend you upgrade to something like quard core CPU and GTX 570 and buy yourself a SSD install the game on it and have fun. Have a fair amount of Ram let's say 8GB would also help. Proper ingame settings also help a lot. No need to set everything ultra high. I set everything high except AA and other eye candy stuff I set to normal running in 1900x1200 res:bounce3: