here's a link to some good stps to follow to avoid image buffer errors:
http://generalspecialist.com/2006/11/avoiding-after-effects-error-could-not...
it's a little old, but the settings will still apply to cs3... one other thing that you should do for cs3, is try disabling multiprocessing (preferences>multiprocessing, uncheck the option to render mulitple frames simutlaneously). with only 4gb of ram you won't be able to effectively use multiprocessing anyway, and this will give more ram to the forground process, which is what you need.
also, with 32-bit windows, there is a 4gb switch that needs to get set for windows to utilize all 4gb of ram. i don't know the specifics, but search the the cow or the web for 'windows 4gb switch ini' and i'm sure it will turn something up.
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW
