Absolutely, because the SeaSonic S12II and M12II power supplies are designed to be able to deliver their rated wattage (like 520W) continuously as opposed to the rated wattage being the peak capacity.
Future upgrades wouldn't be a problem either because each new generation requires less power than the previous. I mean, the 580 needs less power than the 480, the 680 needs less than the 580, the 780 needs less than the 680, and so on. The same is true for AMD, but 3-digit numbers are easier to type. :)
There aren't many peripherals you can add that would change the power consumption enough to make enough of a difference.
