Two 7950's will easily beat a single 7970. The 7950 is a great card, and has a lot of room to overclock. I run my Sapphire 7950 at 1100/1500, well past the levels of a stock 7970. So basically 1 7950 alone can be overclocked to match or beat a 7970 (stock), two of them in crossfire would be a lot more power than a single 7970 could ever come near.
Unless you're running a 4k monitor, or gaming on multi-monitors (EyeFinity) the two 7950's may be overkill. You may wanna go with a single 7950 for now then add another later when the price drops or you really need the power.