Would it really be worth upgrading my pair of HD6950s to a GTX680? It's either doing that or getting the new Arctic Cooling Hybrid GPU cooler for the 6950s and overclock... But the temperatures now are unbearable (top card getting 84 degrees idle)...
Even a GTX670 is a possible upgrade path... I don't want to go with AMD anymore, just too many problems :L
Performance will be similar, not really an uprade performance wise , but definately will be saving you power, heat and driver issues going with a single 680. They run cool and quiet and if you want more performance later you can always add another.