I'd very much prefer not buying anything used. The MOBO doesn't have to be spectacular or even decent, it just needs to be able to run SLI. Forget the price range I posted earlier and please just list me any bare bones MOBO that will get the job done
This ASRock Z77 Extreme 4 is literally the only socket 1155 motherboard still available that supports SLI:
http://www.newegg.com/Product/Produc...82E16813157293
If this is the way you want to go, don't count on it being around that much longer.
As others have suggested, a better path is to sell your 760s and buy a 970. The overall cost will be the same, since you can actually get something for the 760s. Your current motherboard, on the other hand, has no value. And the 970 will provide a better overall experience than 760 SLI.
Not to mention that the upgrade process will be infinitely easier switching out the video cards than switching out the motherboards.