The generally agreed upon number is that above 30 fps, it will look fluid (you won't be able to distinguish individual frames), and below 30 fps, it will start to look jerk-y and you will see individual frames. 30+ is a good baseline. For strategy games like SC2, I'm willing to go down to 20+ without really sacrificing imo.
If you're using Steam, there is no keyboard shortcut. You have to go into the keyboard options, click "advanced", and check "enable developer console." Then press the tilde key (~) to open the console, and type "cl_showfps 1"
Mac frame rate commands are game specific for the most part. WoW is ctrl-R iirc, so it is not even consistent across blizzard games.
30 FPS is adequate - but if the FPS dropps you will notice considerable lagging. I like to have at least 45 FPS so if the FPS drops a bit it will still be around 30 FPS