2 GTX TITAN X SLI OR 3 GTX 980 Ti SLI

For gaming: two GTX 980 Ti's in SLI. Diminishing returns: 2nd card 80% FPS increase, 3rd card 30% or 50% FPS increase?, etc.

For rendering: three GTX 980 Ti's with SLI disabled. I'm no animator but a lot applications that take advantage of GPU rendering need SLI to be disabled to work properly. Great thing is that there is a 1-to-1 ratio for each card that you add.