asrock H77M adequate for crossfire

Anonymouselite5

Distinguished
I want to try crossfire but my mobo is an ASrock h77m. It supports it but I think it's a (pci 3.0 16) and a (pci 2.0 16 (4x mode))

would crossfiring 7850's on the 4x pci ruin my performance and not give me performance of a 7970?

Also I have already been told to perhaps buy a 280x but comparing it to a price of a 7850, it's way to much.

corsair gs600 psu
i5 3570