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Forget about 4K gaming unless you have a few (more than two) high-end GPUs as the GTX 980 averaged just 37fps at 2560x1600 while the R9 290X offered 35fps.
Out of interest, I ran some 4K tests using the GTX 980 and we received less than 20fps on average. A pair of GTX 980 SLI cards is only going to net you a little over 30fps on average at 4K.
Those wanting to game at 2560x1600 or the equivalent resolution using multiple monitors could get away with a single R9 290 or GTX 780 Ti/970 but anything slower will require the quality settings to be reduced.