Have you ever tested how much your computer draws at load? Using something like a Kill-a-watt is the easiest. But anyway, a 750W would power your system fine. People tend to over exaggerate what they need. Crossfire 6970s only draw 596W
tested at the wall, so assuming 80% - 87% efficiency, which is about the range of bronze to gold PSUs, you're looking at 476W or 518W full system load with the cards at 100%, which is pretty unrealistic the way Furmark stresses cards. Even though you have a heavily overclocked system you're still going to have a little headroom with a good 750W unit.