How much is smooth enough to play when it comes to FPS?
It's all subjective. For me, it depends on the game. I can't play a first person shooter unless it's running at a constant 60 FPS. However, on games on the other end of the spectrum, like puzzle games, you don't need a huge FPS. For example, when PSP SNES emulators were still evolving, I was playing Puzzle Bobble comfortable at around 10 FPS.