Here's a calculator: http://extreme.outervision.com/psucalculatorlite.jsp
For a basic system (as I don't know what else you will have in it), the minimum wattage is 260W, with a recommended wattage of 310W.
Therefore, you should get at least a 350W, but might want 400-500W, depending on what you forsee in the future.
If you crossfire, it goes up to 370W recommended. So the 400-500W might be the sweet spot, leaning towards the 500W, just for room, and safety. But also, make sure you get a 80+ cert psu from a reputable manufacturer, and you should be all set.