The defaults are probably sufficient. The description in the preferences window is about as in depth as is really necessary.
The human eye cant see anything more than 60fps, and 30fps is considered full motion. In EverQuest, the lower your fps, the slower your character moves (most games dont have this problem, and the problem is that EverQuest does not know how much time was consumed each frame... it just moves you x amount each frame, to the best of my knowledge).
Basically you want high enough framerate in the foreground that it looks full motion to you, and you want low enough CPU time taken by background processes that each session runs well but high enough that those sessions are able to keep up.
For EverQuest, you also have the option of rendering limiting, which has absolutely nothing to do with FPS or your character's movement rate but is often confused for both of those. In the foreground you want this left default, and in the background ... you probably want this default also. Really, the defaults are just fine

Rendering limiting frees up some CPU power used by drawing the game world itself, by way of not drawing the game world on every frame. It will be drawn 1 out of x frames, or x-1 out of x frames (1 out of x is going to free up a lot more).