See following review that can help answer some of your questions. Performance depends on the game. Most of the time the HD 7970 is slower than the HD 6990. However, in Batman Arkham City, the HD 7970 outperforms the HD 6990; probably because Batman does not make use of Crossfire tech.
I have two HD 5870's, how much more performance will I see if I get two HD 7970's over my current setup?
Performance will be close to the HD 7970 seeing that 2 HD 5870s beats a single GTX 580 by a little though.
But speaking of the lag of 1 GB of VRAM in the HD 5870, the poor tessellation and DX11 support, Power consumption in the HD 5870 you might wanna go the HD 7970.
I'd say that you should sell those 2 5870 and get yourself a 7970 when they become available. Just as the member above me said, the 5870's have less vram (important if you want to game @ 1080p with max AA/AF), and their tesselation performanse is poor even compared to the 6900's. So the 7970 is the way to go, even one will be enough, add to that its great overclocking abilities, and you have a winner card