Get openhardwaremonitor (it's a little Freeware Tool, got its own website to download it) and then select ram, gpu core load and all 4 cpu cores (select the checkbox next to it) and then go into the settings and select "show plot" and the direction where you'd like to see it.
Now you'll see graphs drawn for everything you selected.
If ram goes above 8192, you'll need more memory. If your gpu core load (graphics card) goes to 99% plus you'll need a faster graphics card and if memory and graphics card are fine you are probably hitting the single thread performance limit of your cpu. And therefore would need a cpu with a better single thread performance.
If in a rare case you'll see your cpu cores reaching 99% plus and even on 3-4 cores, you would need a cpu with more cores or hyperthreading.
Don't spend money on guessing, experience or whatever. Get a hardware monitor that can draw graphs (if you only know maximum and minimum you'll get false data due to loading screens etc since you won't know when the maximum happened!).
That way you'll know black on white what your limiting factor is.
I can tell you from my experience with an I7 2600k, 16gb and a 1070, that with the current sims it's basically my i7 all the time that's limiting. Ram usage barely goes above 6-7 GB and my 1070 isn't maxed out apart from rfactor 2 in the night with rain.
One important note: reading out single thread limits of the cpu is basically impossible. The cores won't show it, nothing will show it.
But gladly you can do it the other way around:
Gpu not maxed out (which is reliable info) and ram not maxed out = cpu limiting
