AE generates RAM Previews up to the point where it runs out of RAM to use. You don't have much memory, so that's all the preview you can get. For longer previews, reduce the comp window's resolution to half or less, use the Region of Interest (look it up in AE's Online Help), and Use Shift-Ram Preview (defaults to viewing every other frame).
It's a cinch you can't use multiprocessing, so if it's on, turn it off.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA