It looks like the Inno3D GTX 470 is a really nice overclocker if you don't mind a card that takes up three slots. http://www.hexus.net/content/item.php?item=25123&page=1
When you overclock it as much as Hexus did (comes at default clock) the gap between it and the 480 is really small. And it seems to run much much cooler. Not sure what's the price on these and how available they are but if they're around the same price as a 5870 I think it's still definitely worth considering.