Then that means your PC is running to the maximum potential it can. This also means if you wanted to upgrade the GPU, you would need a stronger CPU to get more performance from it, at least in this title.
Not all titles are properly coded/optimized so the result you have in Forza 4 might not be the same in all games.
im running an i3-6100 with a 1060 3gb, I have heard that a good FPS boost = upgrading your CPU, is this true? (in the case that the GPU is running at say, 50% and CPU at 100%
Not always. When you get a stronger GPU, people will usually increase the settings meaning the GPU is taxed more than before so you do not necessarily get more frames as a result.
Generally speaking, lower settings and resolution results in more frame rates but to push more frames, your CPU must be able to handle more and more work which a 6100 might have problems doing due to having only 2 cores even if it technically has 4 with hyper threading.
This does not mean your CPU is bad or anything but just something to keep in mind.
i know the CPU isn't bad, but at the same settings, with an i7- 7700 or whatever would there be a FPS incerase? (or would the PC be able to handle more)
Since your GPU is running at 100%, it would not increase your performance. If your GPU was running at 60% though, you would definitely gain some performance.
ok, what about CPU intensive games?