i dont think a single 980 would be better than dual 680's, pretty much on par.
But lets imagine for a second your 3770k does bottleneck a 980, what would you upgrade to thats better for gaming? a 4770k thats 5% better with the cost of motherboard upgrade to boot? certainly not cost effective for a maximum of 5% (best case scenario in gaming).
What resolution do you play at? if 1080p, i dont think there is a point getting 2 980's. a single would give you a minimum of 60fps in all games at 1080p ultra detail.