What would you be upgrading to?
Your board is not really a SLI certified board - most likely because you have only 1 PCIe x16 (3.0) slot with one PCIe x16 (2.0) slot. I'm not sure if SLI would auto-downgrade both to 2.0 slots. The only way to enable a SLI setup would be either both slots being capable of PCIe x8 (splitting the channels via switching) or adding a PLX chip which would multiplex it into dual x16s.
An upgrade to something with full SLI (either dual/triple/quad with the proper setup) WOULD be an upgrade, but it would be expensive. You have a good board and a decent GPU, it would be better to upgrade the GPU than to change the mobo.
Upgrading the mobo to something PLX'd would cost $300+. Then another GTX 970 would be another $300ish. Instead, get the new Gigabyte 980ti Xtreme for $680 or so, sell the 970 and that 980ti will be significantly faster than the SLI 970's for a total investment of about $400ish depending on what you can sell the 970 for.
That makes more sense than tearing apart your entire rig.