the idea of use a big psu is to be ready in the future, just in case you add another video card
also, if a psu is under load, the fan will spin and make more noise, so a low load on a 750 psu, 200 or 300 watss, well, is like nothing to it
also, it is more common to see a high quality psu, gold or silver on the 750 to 1000 w range than in the 430 to 550 watts range that is common to find them in bronze or even without any rating