4K now is the 1440p of 4 years ago. You're really looking at SLI for 60 FPS, and even then, not necessarily ultra in all settings, at least not for fully locked 60 FPS.
That was me back in the day. Now a days a single igh end GPU is enough for 1440p, and that will eventually (maybe even as early as next year) also eb enough for 4K.
So yeah, two GTX 970's and a latest gen i7 processor is your ticket (980's if you want more vRAM headroom for a little bit of AA), so about at leat $800 in GPU's and CPU and RAM. You'd still need a mobo and case and hard drives and OS though (more if you go GTX 980's).
Basically, at least, personally, I'd wait until a single GPU can do it, or GTX980's fall in price. By which time Gsync + 4K will also be cheaper.