Quote from Forbin :

There's no way in hell he's exceeding 400W. Only the hottest, most power hungry systems these days pull around 300W under full load:

Yep. Exactly. In fact, if the PSU you are using is rated way higher than what your system actually needs, in terms of wattage, the PSU is working at LOWER efficiency.

http://www.silentpcreview.com/article276-page1.html