If you're trying to edit in AE, that's a very bad decision. AE is just about the worst video editing application on the planet. I suppose Microsoft Word might be worse, but they're close.
Use a proper video editing application: Premiere, FCP, Avid, that Sony thing, heck, even Windows video editor. They're all head & shoulders above AE.
Footage window is for viewing source footage.
The composition window is for viewing your work.
Ram previews are necessary to view the composition window in real time with audio.
The length of the ram preview depends on the resolution of the composition window and your available ram.