Well, if you are gaming at 1080p, then I'd toss the 770 or even a radeon 280x into the mix. Either one will run 1080p games on Ultra settings just fine, no need to jump up into the 780 territory. For most games though, dual 660's would work well too, provided you have a PSU that can handle it.
So, either stick with another 660 and your upgrade path is done, or buy a new 770/280x and possibly upgrade in the future again.