One of the hard things about building a new computer is figuring out how big of a power supply you're going to need. Too big and you're wasting money and too little and you're stuck having to return something and get a new one. To help get around this the fine folks at Newegg
have put out a handy dandy calculator
which allows you to put in the components you have(or are buying) and they'll spit out a recommendation of how much power you'll need to run your system. It's a nice little tool and should save you an extra trip (or two) to the store.
Thanks to Lifehacker
for the find (and for the other tips)