One problem is that it seems like you are looking for a specific FPS or refresh rate that is "the" minimum for perception of fluid motion an the simple truth is that there isn't one. Every person has a different abiltiy to perceive motion and therefore will have a different threshold for seeing something as fluid versus stepped. Case in point, before refresh rates on CRT's could be adjusted (increased above the fixed 60 Hz that AC power is supplied at) some people would experience headaches because they were sensitive enough to perceive at some level the fluctuation of the screen. What was the bigger element of this phenomenon was the conflict between dis-synchronus "stobes" of the CRT and the flourescent lights. Is there a general number??? I don't know. But as I said, there certainly can't be one where you can guarantee a fluid experience for everyone.