Firstly, the cards I've owned:
-4850
-4890
-5850
-GTX470
-GTX480
I'm by no means a fanboy.
Simply checking a few review sites and comparing the GTX480 against the 4870X2 (I guess there'll be a few differences) but the 480 beats it in pretty much everything - making it the better card. (Besides, having a single card is almost always preferable to having 2 cards, where most people would take a small performance hit to have a single card - and thus simplicity, without having to worry about SLI/Xfire drivers - and trust me, I've been down the SLI road. Didn't end well, hence the GTX480).
Quick link:
Clicky - not the be-all-and-end-all but it's a quick look over the cards.