If you want to have fun
Check this out:
eXtreme Power Supply Calculator Lite v2.5
It's a psu calculator, don't take it as 100% reliable, but it will give you a great idea of your power consumption.
With the information given, (with adding some fan and usb device), you take 441 on peak load.
As you can see, it's far less than 750w.
If we go SLI (285X2) we have jumped to 581W. But for security and long lasting for the current build 650 is excellent (not the SLI one)
Personaly keep a margin of 100W for ''further'' upgrades.