Originally posted by RAART Th video card alone draws 230w... CPU is I think around 105w. That.s already around 350w and do you really get 100% output from power supply.
Yes, you do get 100% output from PSU.
If you're thinking about the rated efficiency, an efficiency of 80% means that a (1000W for example) PSU will be consuming around [1000/.7=] 1400W at full load (they don't have max efficiency at full load), and around [900/.8=] 1125W at 90% (900W) load.
But what you should look at is really the rated power. Don't be fooled into buying a 750W (600/.8) Bronze 80% PSU if you decide you need 600W (400 + smart over-provisioning).
That said, nothing wrong whatsoever with buying a 750W PSU when your estimated power draw is around 400W (your example above). That'll keep it running cool and extend longevity.
But 3 times the estimated power draw is unnecessary, and won't give you more reliability.
P.S.:Higher efficiency rating is good for your pocket, and maybe as an indication of component quality, but you can also find some forums with advice on best/worst brands (no comment on your choice of brand, only the power)