<< Nope, wrong. We see at 24fps. That's why movies played in the theater are 24fps. Television is 30 fps.
Why do you want 120fps instead of 40? Lets see... how many degrees are in a circle? 360. How many frames would it take to render a circle so that your eye could not tell that it was dropping frames? 360.
If you're playing QuakeIII and you're getting 40fps, and you spin around really fast (180degrees in 1 second) to nail the guy behind you, what do you get? Well, you need 180 frames to render the full 180 degrees, but you're only getting 40. So what does that leave you with? It leaves you with your computer rendering 2/9ths (22%) of the number of frames needed to make it look smooth. When you turn, it will look like your computer lagging more and more. It's not lagging. It's just dropping frames that are needed to make it look smooth.
Now... lets say that you're running 120fps. That means, in order to spin around to nail the guy behind you, you'll need 180 frames to render all 180 degrees, right? Well, with 120fps, turning 180 degrees in one second, you only render 66% of the number of frames needed (180). Not bad, but would be better if you had more fps. Is it going to matter at this high? Depends on how fast you're needing to turn around. It's easier on the eyes when you're just running forward or backwards, because the fps needed aren't as high. >>
Actually, the eye can see far more than 24 fps. The only reason movies are so low is that they use motion blur. If it wasn't for the motion blur, movies would appear extremely choppy. As for your 180 degrees argument, it is laughable. I don't know where you came up with that 1 frame per degree argument, but it sounds to me like you pulled it out of your @ss. If you are spinning around extremely quickly, the game doesn't attempt to render every degree. Just like if you are spinning your head really quickly you don't focus on every degree. The reason you may seem to notice a lag when you are spinning around has far more to do with your input devices than frames rendered.