For maximum settings, that isn't a bad framerate. If you want to identify a bottleneck, try running the task manager in the background, keep it at the performance tab. Alt-tab out of the game and see what the load on your CPU cores is at. If all 4 cores are running at 100%, then the CPU is your bottleneck, and you would benefit from upgrading your CPU, though the Phenom II won't be a whole lot faster than your Athlon II.
If the CPU is not running at 100%, your GPU is probably the bottleneck. You don't have too many single GPU options above the 6950, only three single GPU cards are more powerful, the Radeon HD 6970, GTX 570, and GTX 580. The performance gains you would get from those cards over a 6950 would not be worth the extra money you would have to spend. If your board supports Crossfire, you could add another 6950, which would give you a good performance boost. A 6950 Crossfire would definitely be bottlenecked by your Athlon II to some extent though.