Ahhh. the framerate thread. Beware my long postij.
Just remember that there is a big difference between the framerate you see on your screen, a timedemo average, and the "minimum" framerate you will see.
Minimums are really what kill the fluidity of a game, and usually occur during the most fun (i.e. action packed) part of the game.
Now, there are typically 3 views on this... and it all depends on your own opinion, but I've experiences that the opinions tend to fall into 3 categories. The first say a timedemo of about 30fps is fine, the second say a timedemo of 60fps is whatcha need, and then there are the people who insist on +2000fps.
There is no correct answer, of course. It's all subjective. If framerate remained at a constant 30fps the game would be smooth. A constant 60fps has been argued (3dfx wrote an excellent article about it but it's not posted anymore) to be the best framerate for 3d games.
Depending on the game, however, the minimums can be severe (or not). A flight simulator generally doesn't need the high fps of a first-person-shooter game like Quake3 or UnrealTournament. The latter at 30fps aren't alot of fun. This is why many hard core first-person-shooter (too many fps's around here) multi-player gamers tweak their systems such that they spit out hunderds of fps. This is in effort to make sure the minimums are never below 60 (and is the case of quake engine games, better movement is possible with more fps).
There is no right answer. More is better, there is a point of diminishing returns, but it's different for each person and each game.
Here is my realworld example. Tbird 1Ghz, Geforce2 GTS (everything overclocked to the brink of stability). In quake3 if I run at 1024x768x32 w/S3TC, my timedemo's are 100+ fps and the fps counter can see ~140's. When playing on servers, the game often gets choppy when alot of peeps are in the same room shootin, I've seen framerate drop as low as 35-40. 800x600x16 timedemo's in the 150's, and the fps counter can go into the 200's, and it's always smooth.