Bouzouki is right. I have a 6600GT SLI set and a 6800GT SLI set, and one 6800GT is in general a better solution than 6600GT SLI.
The 6600GT SLI is not much worse, but the 6800GT wins more benches than it loses, unless you have a really high end CRT and are looking to game at 20X15 0X16X.
The 6600GTs do have Purevideo WMV9 going for them if that interests you.