I you wish to move up to 4K, then the graphics card upgrade will be most important, since an increase in resolution typically doesn't increase load on the CPU, but rather on the graphics card. Generally, the CPU will be doing the same amount of calculations at 4K, while the GPU will need to render 4 times the pixels of 1080p, or 2.25 times the pixels of 1440p. Even a 1080 Ti can't manage to maintain 60fps in some of the newer games when set to max settings at 4K, and that will only get worse with some of the games coming out a year or two from now, so in my opinion 4K isn't really all that practical yet.
Now, in the case of 1440p, there will be much less demand on the GPU to maintain at least 60fps, though if you want to push higher frame rates in newer games on a 144Hz screen, you will still want a faster graphics card. You might also want a faster CPU as well though, since at higher frame rates, CPU performance can also become a limiting factor, depending on the game. Ideally, you would look for something with not only more cores/threads, since some newer games are starting to make use of them, but also with higher performance per core, so that the CPU is less likely to be holding back the graphics card's performance when in the 100+ fps range. Something like a six-core i5-8000 series processor might be a good option since they not only have more cores, but also offer notably more performance per-core than your current processor. The 8000 series i7s also add Hyperthreading, but that doesn't really do much for today's games, and it will probably be a number of years before games benefit much from having more threads than what the six-core i5s offer.
Of course, you would need a new motherboard and RAM for those processors, which might make upgrading the graphics card within that price range impractical, at least without selling the old card for a reasonable amount. Upgrading the graphics will probably be most important for most games. However, you could probably hold off on a CPU/Motherboard/RAM upgrade for a while, and maybe just go with something like a 3000 series i7 and your existing board and RAM, as was previously suggested. It won't have much higher performance per core, but at least the extra threads could help smooth things out in some games, and even if some games are CPU-limited and don't get much more than 60fps, they should still be rather playable.