Here is a nice tool for finding out how much wattage that you are going need for a build
http://extreme.outervision.com/psucalculatorlite.jsp
My general rule is to take whatever it says and add 25% and that gives me the size of the PSU I am looking for. Also very important is to make sure that you selected a quality PSU. Antec, Corsair, SeaSonic and XFX are just a couple good names to look into.