1. You would use a 320×240 composition if you eventually wanted to render that comp to a 320×240 movie. Is that your intent?
2. If it is your intent, then you can scale down the 720×480 movie to fit the 320×240 comp. The quick shortcut is ctrl+alt+F on Windows, cmd+opt+F on Mac.
3. If it is not your intent, and you eventually want to render your movie at 720×480 (DV), you should not use a 320×240 comp size — your final render will appear pixellated. You should use a 720×480 DV preset size (under the composition settings) then set the magnification button at the bottom of the comp window to 50% and the resolution button to half. These will not affect your final render, only the viewing size and res of your comp window.
4. Please read the manual. It’s apparent that you’ve jumped into AE without reading the manual, and AE does not reward such a choice. If you don’t want to read the manual, get Adobe’s Classroom in a Book to take you through your first project. Then get Trish and Chris Meyer’s Creating Motion Graphics, Volume 1 book.
Steve