I'd also strongly urge you to carefully consider the value to SLI today.
Even though there was a resurgence in it's popularity over the few prior years and theoretical improvements at the API level with Vulkan and DX12, a significant number of gaming titles are no longer compatible with SLI. I'm frequently annoyed to find that titles I should be able to enjoy in 4k with my pair of GTX 970's fail to support SLI, disallowing performant play. I believe much of this is related to the Unreal Engine 4 generally making SLI support difficult and more generally tight interactions between GPUs being needed for physics calculation.
In any case I'll forewarn you that the days of being able to slap multiple cards in place to see high performance may have come to an end for a while.
Even though there was a resurgence in it's popularity over the few prior years and theoretical improvements at the API level with Vulkan and DX12, a significant number of gaming titles are no longer compatible with SLI. I'm frequently annoyed to find that titles I should be able to enjoy in 4k with my pair of GTX 970's fail to support SLI, disallowing performant play. I believe much of this is related to the Unreal Engine 4 generally making SLI support difficult and more generally tight interactions between GPUs being needed for physics calculation.
In any case I'll forewarn you that the days of being able to slap multiple cards in place to see high performance may have come to an end for a while.