You'll want as much GPU power as possible. A 3x1440 setup is 11 megapixels, 25% more than a 4K screen, and you probably already know just how intensive 4K is from any recent GPU performance review. 970s are certainly quite capable, but if you can afford it, definitely go 980s as they are generally ~15% faster than 970s at the same clockspeed and are just as capable of hitting high (1400-1500mhz) overclocks.
-Edit-
Actually, I forgot to mention, if you don't already have a machine setup for this, you should probably go 3-way 970s as they will provide even better performance that 2x980s for nearly the same price.
You'll want around a 750W PSU to run either 2-way 970s or 980s, or at LEAST 1kW for three-way 970s. More capacity may be required if you plan on reaching high overclocks on both CPU and GPU(s).