You've got crossfire, but it's x16, x4, which is really not very amazing.
You don't even need the same model, actually, just the same chip; you can pair a 7950 with a 7970. It's pointless, since the 7970 would downclock to match the 7950, but hey.
Only thing to pay attention to is the new boost versions vs the old non-boost versions; I don't believe they play nicely together.