Originally Posted by micatske
I see their support sheet. But the GPU brand require 600+W for GTX770 and 500W for GTX760.
They say that to cover PSUs with substandard 12v rails. If you're not aware - it's more than just the overall wattage of a PSU that matters. PSU power ratings are calculated based on the maximum they can supply across all
the voltage "rails" (12v, 5v, 3.3v, etc.). Used to be that 5v was more important, but these days the 12v rail is by far the hardest stressed, since both the CPU and GPU use it. Unfortunately, there are some unscrupulous manufacturers out there that put an inflated 5v rail in their unit (for cost reasons, probably) so they can claim "500w" on the box, when in reality, the 12v rail (the one that matters most) might only deliver as much current as the one in a better quality 300w PSU.
Most good PSUs are able to deliver 90% or more of their power rating on the 12v rail. The ST45SF-G, for example, is rated at 37a @ 12v, or 444w -- very close to its 450w rating. The GTX770's TDP is only 230w, and your CPU's is probably no more than 75w (assuming Intel). The rest of the components in a system aren't more than 10-20% of the overall power draw. Realistically, such a system probably won't draw more than ~300w -- which a quality 450w unit will handle, easily.