A GTX 680 is just about double the fps in most games
http://www.anandtech.com/bench/Product/519?vs=555 So even in absolute best case scenario for GTX 470 SLI it will probably be a tad but slower than a GTX 470.
A GTX 680 is about $465, and you can pick up used GTX 470 for $100-150. Assuming you can sell your GTX 470 (after fees/shipping etc), you could probably pull in about $80-120 to offset the cost of the GTX 680.
Personally, I'd go with the GTX 680. Sure, you're out of pocket more money initially, but when the next generation of cards launches you'll have something to sell that is still worth something. With two GTX 470s, you'll just have two cards that aren't worth much.
Of course there are other good options such as the GTX 670 or 7950 and 7970.