The 7970 would be a horrible investment for bitcoin mining. Not even a good upgrade for gaming if you already have a decent card. Games only run so well.
Well, think of it like this. If the best you can get on a 5870 at 188 watts at stock clocks is about 380 give or take (not on SDK 2.1, of course), and the best you can get out of 7970 is a theoretical upwards of over 600 at 200 or 250 watts, and you either want to stick with a single GPU (you mine on the side), or you're at a slot premium (a bunch of mega miners at 4, 5, 6, whatever cards per box, and you have a couple boxes) and it is cost prohibitive to buy more boxes (or you ran out of power and don't want to install more circuits), 7970 still wins, especially once it starts dropping below $400.
I mean, its either that, or you sell your soul to the FPGA people. FPGA just isn't right for everyone.