Considering efficiency and all, the computer is drawing no more than 420W from the PSU...
And companies have stated that they will try to increase efficiency. There is a good reason
So it IS safe to say a 650 +/- watt supply is enough for qx6700 and 8800gtx sli, and it IS safe to say a 500 is good enough for most other setups...
Here, Nick's system is not close to maxing the PSU out. Hence 500W IS enough.