I actually had 2 x GTX 260 Core 216's in SLI with an i5-750 @ 4Ghz.
I then installed a GTX 470.
Both benchmarked identically.
The GTX 480 should theoretically perform better. As would the GTX 580. Honestly though, the performance you have is pretty darn good as it is. http://www.tomshardware.com/forum/292068-33-benchmarks-core