Haven't read any of the posts in this thread but here's my two cents from personal experience.
2k and 4k look great, but aren't worth the low fps and horsepower required to run it well. Get a 1080p 144hz monitor, it will change the way you enjoy games. Now I just strive for 120 fps in all my games because its so good looking.
I have a GTX 970 and it works great for 1080p, but it doesn't maintain anything above 60fps well without sacrificing graphical fidelity. So I need to upgrade.
However, I would wait for the new GTX 1000 series to come out and then upgrade to a GTX 1070 or 1060. I personally am saving for the GTX 1070 to use the full potential of my 144hz monitor, hopefully. I can never go back to 60hz again. Maybe i'll upgrade to a 2k 144hz monitor when GPU's become more powerful and cost effective to run in a few years. Just make sure to go ASUS with your monitors, imo.
I use an ASUS VG248QE, it was the best purchase I made for my gaming rig.
http://www.newegg.com/Product/Product.aspx?Item=N82E16824236313