Not that much. Really, just make sure that it's rated bronze or better and you should be good.
Quick video about it. https://www.youtube.com/watch?v=dOXTZizoknc
*Figured I'd add in that you could have a gold rated PSU that's not as well made as a plus, so really it's a mix of "rating", brand name, and looking for reviews that disassemble and test the PSU to insure that its parts are good quality. I'd take a bronze rated "SeaSonic" over a Gold rated "RaidMax" any day... (Is there such a thing as gold rated Raidmax?)