Vectorized :
Thanks for answering me guys.
I will pick a 2K monitor as future-proof one. I will update my GPU when a one that will be worth buying than a GTX 780 Ti will come out.
There are no 2K monitors. 2K resolutions are video standards that are a little wider than 1920x1080.
In this case the "2K" is the horizontal resolution rather than the vertical.
FHD - 1920x1080 - Sometimes called 1080p, but this is actually a video standard
QHD - 2560x1440 - Sometimes called 1440p using a similar convention to 1080p
4K - 3840x2160 - If we followed the same standard as 1080p and 1440p, this would be 2K but the name uses the horizontal resolution instead.
These are the 16:9 resolutions. You sometimes get 16:10 resolutions where the vertical resolution is a little higher, e.g. 2560x1600 for QHD.
You are also starting to see some 21:9 monitors that are wider than a standard monitor and have a higher horizontal resolution, e.g. 3440x1440 for QHD.
QHD monitors are the best compromise between performance and quality at the moment for games. 4K is still too much for most setups, and 4K monitors have various limitations.