“600 to 700W units seem to be the sweet spot for high end systems, they offer enough headroom for some serious overclocking, but not so much so that the extra premium for the unit was an absolute waste of money (1kw units anyone?). In this review we'll be looking at two popular units - the FSP FX700-GLN Epsilon and the Thermaltake Toughpower 650W. We’ll be testing them under multiple conditions with X1900 CrossFire and a high end Core 2 Duo setup (X6800). ”

http://www.pureoverclock.com/article638.html