Could you notice a 40fps game?

I agree you can tell the difference - e.g. on the Amiga, could always tell when a game was scrolling or updating at 25fps rather than 50, or 30 rather than 60, because there is a noticeable seeming `overlap` of previous position with new position which is much less when the framerate is higher. You've only got to look at a tv signal at 25-30Hz and things scroll fast, it becomes more of a blur, whereas higher hz minimizes that. But I'd say 25fps is the absolute minimum for any game.