I have i5 3450 im planing on getting rx 570,I saw a bunch of benchmarks at 1080p at 30-40fps. will 720p improve that or procesor is holding the fps improvement?
That would depend entirely on the game in question. Each will have different CPU requirements.
Running at higher resolutions will put more proportional burden on the GPU. So if you do buy this, when you are testing it out, try raising the settings. If you see no FPS improvement, means it is the CPU, then you can try to get as much graphical detail as possible. If you get more FPS with lower settings, and that is your goal, hooray.