Looking for an NVidia GPU that can handle my 27" 2560x1440 display playing AAA games

When I built my desktop, I initially got GTX 960 2GB, thinking that it'd be good enough, since I only had a 1920x1200 display, which worked out okay and I could play all the games I wanted. But then I bought 27" 2560x1440 display, and now I can't rea

But what I didn't realize, that when upgrading to 27" 2560x1440 display, which is amazing, except I can't play most games even on mid details on full resolution. So now I'm looking to upgrade the GPU to handle this new beast of a display.

I should probably note that my motherboard is MSI X99A SLI PLUS, so I guess SLI is an option, but I have absolutely zero experience with it.

I don't want to specify a budget, since on one hand I don't want to buy something too expensive, such as 2x Titan X, but on the other hand I already have a shitty graphics card that can't handle my resolution, so buying another shitty graphics card won't really be an upgrade.

Would it make more sense to buy something like 2x 970, or rather a single 980 or 980 Ti? Or would even a single 980 Ti be enough?

I'm not really sure how much SLI helps, and by using 2x 970 I could expect double the framerates than a single 970 (to put it simply)?

For comparison, my laptop with GTX 970M seems to handle almost everything on high/ultra settings on it's FullHD resolution, and even though the GTX 960 seems to do quite a lot better in bechmarks (http://videocardbenchmark.net/compare.php?cmp%5B%5D=3114&cmp%5B%5D=2981) it performs incredibly poorly on high resolution than the 970M on a lower resolution.

Last but not least, I'm not looking to upgrade immediately, and I wouldn't mind waiting a few months if that means I can get a much better GPU. Even if there's a perfect buy right now, I'll probably not upgrade until at least a few months anyway.