The i5-3570 is a 77w TDP chip, and the GTX 670 is a 170w TDP graphics card. Throw in all the other hardware, and that looks just about right. Everyone over-estimates what size PSU they truly need. You could easily get away with a high-quality 450-500w PSU and save some money.
it not the total output you have to watch out for it the amps rating on the 12v line. 300w is 25 amps. with any power supply you dont want to to load it more then 70 percent of it rated output..anymore and it can ripple out or over heat and have voltage drops.