If you CF HD 7870 with HD 7850 you'll end up having 2 HD 7850s in CF and of course they will give you a great performance over HD 6970. The new HD 7xxx series consume less power and run much cooler than the previous one.
Also, a single HD 7850 is in the same level of HD 6970.
Btw any reason why you are not considering the GTX 680 from nvidia? It's only 30dollars more and significantly more powerful
well, the main reason being is i live in the netherlands were the price differences are a lot higher in most web stores i find. Ranging from 100-150ish for some reason. But the GTX 670 does look pretty damn good