Calcs are a bit hit or miss. But in this case you can take a look at Anand's benches for a good idea of power consumption. At 100% load the 470 system has 408w consumption and the 8800 system has 278w consumption. So 686w, but that's including a second physical processor as well as a second set of all the miscellaneous smaller power draws in a system, so you're probably closer to 600.
Not to mention Corsair PSUs are very well built, so you could probably go a bit over 750w on the thing and still be ok in a pinch.