You can crossfire different cards, AFAIK.
Eg: 5770 + 5750, 6790 + 5770.
50 bux shipped is a fantastic deal.
I don't have the link on AMD's site, but what I read said that only cards in the same model/line will work together. Ie 5770+5750 ok. 5770+ 5500 no. please correct me if I'm wrong.
Considering I picked up a 4870 for the same price here a while back, this is a great price for it.