You should list your whole system spec so someone can answer more accurately.
Playing at lower resolution, it put more work on your CPU. At higher resolution, the work shifts to GPU.
Yes, it will help to upgrade from i5 6500 to i5 8th gen at 900p. However, you should think about upgrade your monitor to 1080p first rather than i5 8th gen CPU.
To upgrade to 8th gen i5 from 6th gen you will also need to upgrade motherboard which is not worth it because the gains you get is minimal. Instead save more and go for new gen GPU when launched that will improve your gaming experience a lot.