In all fairness, the NEED to add a second Graphics card is probably very minimal at the present time.
I personally own a 560Ti and I can run Crysis with everything maxed out with 40 fps. Im not sure what more than this you would want. Games like Call of Duty and Bioshock 2 all run at 100+fps meaning theres no reason at all to add a second card because the increased performance wont make any noticeable difference.
If you sell your 560Ti now, and buy a single 6950 - your going to be getting almost exactly the same performance. Which is essentially, pointless. The only reason you should consider buying/selling any components is if your seriously considering adding a second card.
If you are seriously considering adding a second card, if i was in your situation, id change the motherboard, and buy another 560Ti. The "weak" point of your build is obviously the motherboard, so buying a better one cant be a bad thing.
In terms of cost-effectiveness it would be cheaper to
Sell motherboard, buy new motherboard, buy another 560Ti.
Than it would to...
Sell 560Ti, Buy 6950/6970, Buy another 6950/6970.