I don't have any problem with out of memory errors anymore because I followed the instructions in this thread.
http://forums.avsim.net/dcboard.php?az=show_mesg&forum=121&topic_id=399256&mesg_id=399256&page=


This will work for FS9 also but you must have 2Gigs or more of RAM.

Also go into START>RUN and type in "msconfig" without the quotes and check how many programs are unnecessary under the "Start Up" tab.  Even if you close all of the programs under "Services" or "Start Up" it won't hurt your computer and you can narrow down any out-of-memory problems.  Just add back important programs and leave useless stuff like "Windows Desktop Manager" off while your running your game.
My FS9 and FSX are running much better now with no OOM errors