It won't work, there's only a few cases where you can crossfire a xx50, and say a xx70, I think it was with 56xx's or something. Bottom line, no you can not with those two cards, do you already have a 7770? An upgrade to a 7790 isn't much of an upgrade at all. I would hold onto your money for a bit and step up to at least a 660, or a 7870/7850.
Thanks, i'll do that!