You're gonna sli/crossfire right away or later on ? I mean if you're gonna sli now it's better to get a higher end single solution than the sli way.
660 is better than the 7850.
7870 is a bit better than 660 and will drop in price maybe, when that one becomes cheaper or same price than the 660 that is the better choice then.
Difference is now $10, so . . . ( cheapest 7870 $240, cheapest 660 $230 )