First things first, I know there's a calculator out there which tells you how many watts you will need to run a computer, but I found it quite confusing, and it didn't seem to give me the information I was expecting (I know nearly nothing about PSUs).
Okay, so, here's my set up excluding a case and a PSU):
i wouldn't go lower than a 550w from a good manufacturer, however I'd just get one of the around 600w ones. Look for antec, corsair, seasonic, or silverstone. A psu is the last place you want to cut corners, poor system stability or worse can happen real easy with the bargain ones.