So the system I'm planning to assemble soon needs about 320W according to my calculations but there's a sale going on at a store nearby that has the Corsair CX750 (750W) for about half price. Is there any reason not to get the Corsair aside from the whole 50% efficiency rule? I'm guessing it won't run up my electricity bill? I want the build done by this week so buying it locally would be plus.