What type of games do you play? And no, well maybe, you will lose like 2 frames with the ghz speed. You could just overclock the other 7970 to the same speed.
I will maily play BF3 and BF4 when it comes out and ill just play any game i can get,it all depends on how i feel at the moment, i like Free to play games so i will most likely play a lot of MMORGs.
If you're going to be playing at 1080p on a single monitor setup, you have a perfect build for it. The 7970 is an excellent card and will be able to handle future titles pretty well. And it can overclocked to match a Ghz edition card easily.
I recommend you stick with a single GPU solution. Crossfire and SLI setups require more power, generate more heat, and there are still a few really annoying optimization issues. The 7950's would be overkill IMO. Not worth spending the additional money. Save yourself some bucks and go with your initial build. Its completely suitable for what you plan to do and then some.
Looking back, Izrail has a point. 8350 performance with sli/crossfire is pretty bad, so, maybe swap it out with a 3570k, and an asrock extreme 4, then cfx another 7950 when your original one starts lacking. To cfx, you may need a larger wattage psu.
Topic starters build looks great, the only thing he has to chance is the motherboard, pickup a decent 990fx chipsetboard. like Asus sabbertooth asrock extreme 9 or the Asus M5A99FX pro R2.0.
Amd FX8350, perform better then the i5-3570k in most games. especialy in streaming gaming. so yeah i would defanetly go with the amd fx8350, instead of the i5-3570k in everyway.