Conduct a couple of tests.
1) Make your cpu go slower and see if you still get the same frame rates. Do this by going to windows/power management, and limiting your cpu utilization to some number like 75%. If your frame rates do not change, you are not cpu bound.
2) Reduce the resolution of your game and remove some of the eye candy. If your frame rates go up with a lessened graphics load, it indicates that your cpu is strong enough to drive a faster graphics card. Conversely, if the frame rates stay the same, it indicates that your cpu is limiting you.
Let us know what you see. I think you are relatively balanced now.