Two GTX 260's in SLI is equivalent to a GTX 470. So to get a true upgrade you need a GTX 480, GTX 570 or faster.
I made the switch from 260's to a GTX 470 myself back in July 2010.http://www.tomshardware.com/forum/292068-33-benchmarks-core
Obviously some games take advantage of SLI better than others. So you may get the same performance in some games, or for games that don't SLI well you might see improvements. But honestly, a GTX 470 for the most part is a side step from what you have.
A GTX 570 is faster than the GTX 470, and is probably where you'd want to start looking.