The GTX 980 TI is basically a Titan with less vram, if you don’t need the ram, the GTX 980 TI should give you basically the same performance at a much cheaper price. Don’t use SLI. CPU really depends on how much you are going to be waiting on your CPU to complete tasks, and how many of them are properly multi-threaded. If you have a bunch of single threaded CPU tasks, a i7 would probably be better than even dual Xeon due to the higher clock speeds (this varies of course). If the tasks are completely multi-thread, dual Xeon is definitely worth considering.
UE4 does not support SLI, so it would be a waste if you’re planning primarily for UE4 development. In general, you want to get the best single graphics card you can get. GTX 980 Ti seems like a great choice.
Thanks for the advices, then, with 6 extra GB of memory with Titan X, what kind of applications will take advantages?
If you really wanted to you could max the 12Gb that the Titan X has using UE4, though that’s not typical.
The biggest benefit would be for GPU renderers, like iRay and VrayRT, which could take full advantage of the GPU memory.