The Cinema uses 24FPS
And if your game is only going to be played on monitors which refresh the whole screen instantly ( as projectors do ) and will only be played in a dark room with very bright lights ( creating an "afterimage" effect ) and most importantly if you're calculating real-time motion blur ( movies are made with motion blur for precisely this reason ) then this would be a pertinent piece of information.
As for the original topic, nothing less than 60 FPS is silky smooth as far as I'm concerned, and I'd up that to 75 or 85 if I were using a CRT monitor.