I don't personally have GTR 2, but I have a slightly faster CPU (plus it's dual core) then you and an 8800GTX. In several games, my framerates are still limited by my CPU. At the framerate you're running the game in, your video card is definitely not your bottleneck. I would guess upgrading your cpu first may help, perhaps only having 1 gig of ram being your second bottleneck.
I know in Test Drive unlimited, if you have only one gig, the game has to stream the graphics from the hard drive a lot more frequently, resulting in a lot more hitches and stutters when driving fast.