That will work fine, except maybe if you have multiple monitors crossfire has some issues with that, but you're pretty much good to go. Maybe swap out the 7970s for R9-280Xs because they're almost identical, with a few small exceptions, and cheaper in some cases.
Just the one and I'm mainly building a new one to make use of mantle because currently i have two 6870s and they're getting pretty old now so it's just going to be AMD for this build at least :)
I hate waiting for thing and see no reason to wait, the new AMDs in my price range it's pretty pointless waiting.