This one is a good power supply calculator since it knows all components from the past till today:
http://extreme.outervision.com/psucalculatorlite.jsp. Pcpartpicker is a good site but they only show components currently available.
A little headroom is always good to have, but I recommend considering a couple of things:
First, current components use much less power than their previous generations. Also the efficiency of the power supplies has improved, they produce more power instead of heat.
Second, if your power supply is too powerful, it may cause all kinds of problems. Remember the law of conservation of energy! Although there are mechanisms to adjust both intake and output, power supplies aren't intelligent devices. It's like with shoes: A couple of sizes bigger is good if you have to stand all day, but you don't need clown shoes unless you're Bigfoot.
As SirNuke said, a good quality 600W is all you need for a single GPU system (typo corrected).