I'm wondering then why I'm struggling to beat it. My Q6600 is clocked higher, RAM shouldn't play any part (your 4GB vs my 2GB) and my GTX is at a healthy 655/1075.
Don't get me wrong, I know the GT is a cracking card, but I'm now seriously considering offloading mine and getting a GT. Or could the res play a large part? What size monitor have you got? Be interesting to see the results at the same CPU speed at different resolutions.
clock you shaders up a bit should help.
