SLI isn’t a good option because it’s a bit buggy and there are a number of things that can’t use it, like UE4. Other programs like Maya and 3ds Max can’t use SLI for their viewports either so you want to get the best single GPU card you can. The recomendation of the GTX 980 Ti vs. the Titan X is because the Titan X is not that much faster than the 980 Ti so it’s not worth it unless you need the GPU memory or have money to throw away.
The stuff that will use up RAM–very large projects, like the open world demo for UE4 which requires 24GB to load. Building lighting requires a lot of memory as well, and doing 3D stuff/rendering can also use it pretty easily too.