Hi guys!
The first time I became obsessed about the amount of FPS my machine could handle was when I was playing Halo. Going from my tatty machine to a high end machine, I could easily double my score due to the difference in FPS. This was from 40 to around 80 fps, allows for more precise accuracy, better prediction, etc.
I voted for 80fps simply because it's the ceiling for me in terms of what helps me play better. Sure, I can notice a game dipping from 150fps down to 80 or so, not so much the monitor displaying those refresh rates more so the reaction speed of the games to the input - I'm talking FPS' here, not RTS games, where I don't notice such differences.
It's not so much noticing the amount of frames dipping, that's pretty much impossible imo, unless the fps goes really really low. It's just how fast the game reacts to your control, and obviously, the higher the fps the faster the game will react to the input.