Don’t use that bottleneck calculator website, it is complete misleading garbage.
With your cpu it is going to depend on the game. Some of the latest cpu heavy games will saturate quad core easily. Take BF1 or BF V in multiplayer, a quad core will see dips under 60fps even with a good overclock. In other games the gpu will be the limiting factor.
I would think of it this way. The cpu determines the best FPS you can get in a specific game. The gpu determines the best resolution and game settings you can achieve this FPS with.
8gb of RAM isn’t going to help in newer AAA games. 16gb helps by reducing the caching to the main drive which reduces cpu work and eliminating some stuttering you may otherwise suffer, again it’s game specific. However to avoid issue you want to buy a 16gb kit and not just add 8gb to what you already have.
Personally I wouldn’t go higher than a 1660Ti for 1080p. If your monitor is only 60Hz then even this is a bit over the top but gives good headroom for the future.