This site is being deprecated.
Please see the official X‑Plane Support page for help.
The CPU and GPU are very interdependent, so you would probably need to do testing in X-Plane with the rendering settings options to see what would give you the most "bang for your buck" in your opinion. You can use the steps outlined here in the manual for setting your best rendering options.
However, if you'd like to increase the GPU heavy options while maintaining your frame rate, buy a better GPU. Your CPU is almost certainly not limiting you unless you're running a bonkers high number of objects. Since you have a mid-to-low range GPU from the previous generation, a high-mid-range GPU from the current generation (like a GTX 1070) would be a huge improvement.
This site is no longer being actively maintained.