Try this Powers Supply calculator linked to below, it pretty damn accurate. I used it before I crossfired my cards.
When i did the calculation it indicated a power supply requirement of about 716 Watts, and I have a Corsair 750 Watt PS
In reality you almost never draw the maximum power, because quite simply everything is not running flat out on most occasions. Most games either load up the graphics, and the CPU isn't breaking a sweat, or vice versa.
I have a Watt meter monitoring my system at the wall socket. With a flat out benchmark, with the CPU and both graphics cards running at 100%, the absolute peak wattage was 690 Watts.
But that's a benchmark. The top usage in a game actually hit 590 Watts, but most games average in the 400 to 500 watts area. I can further restrict it if I want by Vsyncing to my monitors refresh rate of 120 hz. So in many games where I may get 150 or well over 200 fps if the cards were to run free (burning power, creating heat and noise), Vsync reduces the load. Or I automatically disable crossfire (via a profile) when the power isn't needed, especially for older games.
Anyway here's the link
http://www.extreme.outervision.com/PSUEngine
Sorry for dragging this thread a little more off-topic. I should have kept the above short and sweet
EDIT: Just to be clear that my power readings are under crossfire, so a single card would draw a lot less.
