Another, slightly different approach to PSU selection,
is to look at their graphs of efficiency vs. load.
Then, pick a point on that graph where efficiency
is highest: in that way, you'll minimize power
consumption in the long run.
Whenever in doubt, a good rule of thumb is
to utilize 50% of the rated capacity of any given PSU.
Interesting... when looking at the Corsair efficiency for a 650 W supply:
The 230 Vac curve is 2% higher than the 115 Vac... Perhaps I should consider running a 230 Vac line to my server to get the better efficiency! Has anyone else out there considered this?