Crossfiring low end cards is not worth it. Just buy the fastest single GPU you can buy.
agreed. not to mention you pay more for a proper crossfire capable motherboard (by proper, i mean one with at lease a 8x/8x slot configuration, not the cheaper boards with 4 x slots which don't perform properly with crossfire). for $20 more than a 7770 you could also get a faster 7790 or a nvidia 650ti.