If you're going to get a GTX460, get a 1GB version. The extra memory might not matter that much, but the 256bit buss width probably will (vs. 192bit on the 768MB versions).
This system would run fine on a quality 500W PSU, so unless your "700W" PSU is a Chokemax, you should be ok.
If you think you might regret not holding out for a BD or SB CPU (either of which would require a new mobo and DDR3 RAM), buy only the GPU now, and see how much improvement you get. If you don't care, or that's not in the budget any time soon, then get a CPU if performance is still low. For gaming, you will not be likely to see any benefit from X6 over X4. A 965 would be decent.