It's enough power, even when you decide to oc the 4870.
I was a little concerned after reading Tom's Hardware review where they suggested that 4870 under full load uses 283W. But on this site, the highest wattage they have for a single 4870 is 183W, which seems more reasonable. http://benchmarkreviews.com/index.php?option=com_content&task=view&id=198&Itemid=1&limit=1&limitstart=11
This site seems to suggest that the 280W number is for "total system consumption" at full load: http://www.anandtech.com/video/showdoc.aspx?i=3341&p=22
If that's the case, I think my intended EarthWatts 500W should have enough juice for the rig.