There are studies on this topic. Until some months ago, it was common to say that anything above 25 fps should be fine. This has a simple reason, starting at 25 hz the human eye is not able to distinguish single pictures and instead sees a motion. Cinama movies run at about 23 fps, I think, and I rarely see people complain about the movie being not smooth.
However, recently, everybody starts saying that the number can't be high enough, that for some reason the human eye sees different when playing a game, and that for this reason 25 fps are not enough. I'm sorry, but I don't really believe it
I'm playing HL mods such as CS and FLF (obviously not the most system-straining FPS) on my 300mhz Celery and a Banshee. Usually I get something like 30 to 40 fps, in wider scenes with many people this drops to 15 to 20 fps. I can play everything fine, the rare stuttering is annoying, but I still 0wn y0u
Of course, people throwing around high frame rates DO have a point. Most likely they're talking about their maximum frame rate. However, the maximum frame rate is rather irrelevant - what matters is your minimum frame rate. Your minimum frame rate should always be above 25 fps (in my case it doesn't). You will get ridiculously high rates when viewing a simple scene (like a single wall) but when the game gets more complicated, the minimum frame rate will take a serious hit.
The minimum frame rate I consider playable is 15 fps. It would be playable, but you (or at least I) would notice a stuttering. My target minimum frame rate when considering resolutions and effects is about at 30 fps.