He has 800W because he has twin 5770's in crossfire which both require at least a 450W power supply in order to keep stable. 800 watts is a good amount of power for the system he is running so there is no reason to upgrade anytime soon unless he plans to upgrade to newer cards.
If you are only planning on running a single video card (460GTX) on an i7 system, I would recommend between 650 or 700 watts, just to keep on the safe side. If you are still unsure about the type of supply to get, you can check out Power Supply Usage Calculators