Using the psu calculator at http://extreme.outervision.com/psucalculatorlite.jsp
and taking a couple of guesses about fans + adding a bunch of USB devices plugged in then it will max out at less than 300 Watts
PSU dont reach maximum efficiency at 50% loading so the draw from the wall will be a little more 360 - 380 watts . Much less at idle of course .
The PSU calculator lite @ extreme.outervision.com recommends the size of PSU
for the system components chosen - not the power draw of those components.
And most PSUs DO
reach maximum efficiency @ 50% loading - with power/efficiency curves that look like this: