You can use much smaller but pricewise, there's ittle to be saved by buying less than 650 watts. If ya not overclocking, I'd recommend the EA-650. If ya are, the X-650.
650 watt power supplies. For comparison purposes , the list below contains performance ratings (10 scale) at jonnyguru.com / prices as per newegg on 12/31/2010 and ones w/ double asteris (**) made it onto silentpcreview.com's Editor's Choice List:
Note: Just because you see a brand name on this list, never make the assumption that their other lines are of matching quality. Silverstone for example has other models w/ 6.0 and 7.0 ratings.