I doubt a 7770 is better than 4850 Crossfire. Just saying.
It depends a lot on the definition of "better". Two 4850's may very well faster than a single 7770, but that's about it.--Wacky Races 2012!
Here's the closest comparison I can find. A 4850x2 is about equal to a 5850 spec wise but take about 10-30% off for Xfire scaling.