Upgrading my GPU

With my i5 4690k, I need some help upgrading the GPU. I only want games to run smoothly. I am now playing in 1440p on a 27-inch 60-Hz display. I can also maintain a reasonably constant 60 frames per second in low settings.
These are my Specs.

CPU: i5 4690k
COMPUTER: MSI Z97 Gaming 5
16 GB 1866 MHz CL10 RAM

May i know which GPU you're currently using ?

Oh i forgot to mention my bad sorry. Its R9 280X GAMING 3GB DDR5

Your GPU is significantly superior and can play all games with a good frame rate. What's going on? It is preferable to go to the Ryzen 5 or i7 family of CPUs.

I know it runs practically all games well, but I still want to upgrade my GPU because it has poor FPS and lags. About the CPU, I have no interest in upgrading it because Ryzen CPUs are more costly than GPUs.

So, in that case, I advise using GTX 1660. This card works well for me, is reasonably priced, and pairs well with an i5 CPU. It uses very little electricity, too.

I appreciate your advice. Further evaluations showed me that the i5 CPU and GTX 1070 perform well together. Which one ought I to pick? a

With your current CPU, both the GTX 1660 and 1070 work well. These can assist you in playing in 1440p at decent settings rather than maximum resolution. Again, the video is accessible here.

https://www.youtube.com/watch?v=IN3MUYniOmg

Before updating any significant components, it is preferable to think about your power supply. Which power source are you employing?

For PSU 

Corsair Builder CX 600 W