1080p is about 2 megapixels, 2K is about 5 Megapixels and 4K is about 8 Megapixels (million pixels).
Pixels are small unique dots, which make an image/ display. The more pixels you have, the better detail you will see (not quality, detail)...
However, more megapixels means more workload for the GPU, so you get lesser frames per second (fps) in gaming...
I don't understand how forum members recommended you 2K display on this GPU. Just for display/light multimedia its perfectly ok, but you specifically said "Games, movies".
Where does one get loads of 2K movies? Did you specifically mention you use Netflix or so? And even Far Cry 3 Blood Dragon (6-8 year old game) won't run on your setup satisfactorily. Then what gaming you plan, GTA Vice City?
To be honest, the kind of medium needs you have, there's no need for a 1440p monitor.. I myself am an avid online gamer who games on 1080p (the GPU is Nvidia GTX 1060 6G).
Most content we see is just 720p or 1080p, including ripped movies. For a GPU, Games run slower on higher resolutions like 2K, and as time progresses, newer games will stop running satisfactorily.
Ask yourself do you have enough 2K or 4K content or access to it? (In one of your posts, you said you're confused about what these terms are).
My sincere money saving advice would be that you sell this monitor off, if you get about 18-20k. Then get a good 1080p one... And get either RX570 or GTX1050Ti with Corsair 450W PSU. That will ensure good future proofing & you'll save some good money too...
Upgrade CPU if its a bottleneck, a latest low end Core i3 is enough. (but not if you're running Windows 7, 8.1 and you need updates. Google about it..)
Alternatively, if you want to keep the 2K setup, go for at least a GTX 1050Ti or RX570, nothing below it. Even that will somehow give sufficient fps (I consider 60fps as a basic standard for competitive gaming & 45-50 fps for casual gaming, & most displays are 60Hz)...
Don't worry, A reasonable portion of the world is still on 768p (1366x768) and even 1080p won't get obsolete anytime soon...