The RAM has a minimal affect on FPS, the main thing RAM is good for is load times. I remember when Battlefield 4 came out my machine was running 4GB of RAM and loading into the game took forever, sometimes even up to 10 minutes. Granted there was a known issue with Battlefield 4 load times.
Point of the story... I upgraded to 16GB memory and load times were significantly lower.
FPS wise though... not a noticeable difference.
My sweet spot at the moment is 8GB, unless I run VMs on my machine...then I want 32GB. But for pretty much everything 8GB is enough.