I just happened to have upgraded from 470's in SLI to a single 680 this week. The 680 is about 30% faster than 470's in SLI, and it's a LOT quieter. That's the big downside of the 470's and why I couldn't wait to get rid of them. GTX 470's are loud with 1, but ridiculous with 2.
So I guess the question is, do you want to spend more for a quieter faster setup?