I can say that going from a dual card setup to a single more powerful card was well worth it to me. Especially an AMD dual card setup. Lots of problems with drivers/game support (especially Skyrim...)
I would say it's worth it. What's the price difference for you between the 7970 and the 680? they'd both be great upgrades for you. 680 is better (if you can get it) but 7970 is a great card too (and overclocks like crazy), so if you can get the 7970 cheaper, that might be a good option for you.