I'd get a 650 watter as it will handle just about any single current GFX card easing any upgrade worries. For twin cards, I put in 850 watters which again will handle just about any current twin cards but the 480/580 and 5970.
With PSUs hitting peak efficiency at 50% load, any premium your paying in purchase cost over a smaller size unit, is easily recovered in electric costs over its lifetime.
For comparison purposes , the list below contains performance ratings (10 scale) at jonnyguru.com / prices as per newegg on 02/26/2010 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.