is anything to go by, you need about 340W @ 90% full load. I usually double it up, round up to the nearest 50W, then do a +/- 50W spread. In other words, a 650W-750W should be good if you don't want to spend that much. However since you never usually have 90% full load a lot of the time, you can drop this down to about 600W and still be good.
Make sure the power supply is at least 80PLUS (Bronze or better preferably) and try to get a single 12V rail. There's nothing wrong with multiple rails, but since video cards require a lot of amps, you have to be certain which rail you're plugging into, otherwise you'll blow it out.