The 980 is actually slightly faster than a 690 (http://gpuboss.com/gpus/GeForce-GTX-980-vs-GeForce-GTX-690), and while it may not be quite as fast as 680s in SLI, the difference would be negligible; for performance at least. Keep in mind, though, that the 980 would give youi roughly the same performance as two 680s while drawing less power than a single 680! I think that's a pretty big deal, and you can SLI the 980 later if you'd like.
All that said, however, just wait out the next card release. A single 680 is more than enough to run Diablo III. By the time you want a new game that demands a lot from your GPU, you'll just get the best card out. Plus you'll have squeezed three generations of use out of your 680, which would be impressive in it's own right 🙂