15 is way too low, above 30 FPS usually feels properly playable but it depends on the game - Crysis is playable at mid-20s. Anything over 60FPS is supposed to be a waste isn't it? Can anyone really can tell the difference between say 80 FPS and 90 FPS?
This is a great (if very long

) guide that answers a lot of misconceptions associated with fps/hz etc
http://www.tweakguides.com/Graphics_1.html
page 5 deals with Frames Per Sec,but the rest of it is also worth a read,the triplebuffering/vysnc parts especially.To many bits to quote but one stand out part relating to this topic is:-
"Maximum FPS
The concept that there is a maximum possible FPS beyond which the human eye can't distinguish any real difference is not entirely accurate. For more details, see this article and this article among the many which refute this claim. In particular the common claim that "The human eye can't see more than 24 (or 25 or 30 or 60) FPS" is completely false, and is partly borne of the misconception that TV or movie FPS is the same as PC game FPS, and partly possibly borne out of a need to justify lower framerates. "
POTTSEY
"Rezident "Anything over 60FPS is supposed to be a waste isn't it? Can anyone really can tell the difference between say 80 FPS and 90 FPS?"
"Anything over 60 is only a waste as very few monitors show more than 60fps. The human eye can easily see a difference over 80fps and can even pick out one frame out of 200. The problem is most LCD screens can only show up to 60fps and any FPS over that are lost."
As pottsey says a few posts higher up,this is the trap a lot of people fall into when talking about 60fps vs 120 fps.Most people with 60 hz monitors have at one time or another run a game with vsync OFF and got silly fps results like 150 fps etc and concluded that they couldnt see any difference between 60 and 150 etc fps.But like this guide says and what pottsey says you havent "seen" the 120/150 fps.Even unsnyced you have ONLY SEEN 60 FPS,even if it is vsynced or not the Difference being when it`s unsynced is that it`s more than likely 60 torn frames.
Anywho if you got a spare 30 mins or so it`s a good read
