By pressing B and N, you can set the in and out points of your preview, if you are running out of RAM(previews are just frames stored in your RAM).
Remember if you change your work area, you will want to change it back when you go to render your final composition.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.