My friend claims that the human eye can not see the difference between (around) 46.7fps and 60fps. Is this true?
Second question is, wether or not that is true, is there a notable difference (again, for the human eye) between 120fps and 60fps. (Allegedly, the later one is only visible because we believe there's a difference).
Take a look for yourself. It's incredibly difficult to tell a difference of 13 fps, but the difference is there.
It actually depends on the person. Everyone has a different framerate they can't see the difference beyond. I know personally I notice it when my games drop to 45.
There is a very visible difference between 47 and 60. Try it with FRAPS, lock your framerate at 47 and see the dodgy framerate when you turn, then compare it to 60 if you feel like proving it.
60 and 120... If you're looking for it, then you're going to probably notice it. Whether it improves your gameplay/reaction etc is negligible IMO. It's nice to have though.