Two 980s will push 4k with decent settings. You won't be able to max out everything, but that's just a consideration. If you go 4K, you'll have the monitor already purchased for when you next upgrade your video cards in a few years. At the same time, there will be better 4K monitor options at that point as well.
I'd save the money now and get a nice 1440p monitor with a single 980. The quality of monitor will be better, and you'll save money on the video cards for now. You might then have some extra cash in a year or two when quality 4K monitors are available for a reasonable price. The 4K monitors in the $500 range are not very good and will strain your eyes pretty quickly.