What is my bottle neck?

[eatmagnetic]

If you don't want to spend that much money, and don't care about other games (like PC games!) - you may just get an AMD HD6850 or HD6870. If you want to perform even better on other games too with your CPU then get a GTX560 Ti or a HD6950 - since your CPU performs quite awesome you will probably only see impacts in picture quality like High-Resolution and AA (SSAA, MSAA etc.) or AF, sine these are only GPU-limited, and you're CPU can't do anything about it.