The Voyager is a multimedia phone and is in no way equipped to handle the functions/applications of even the weakest Mogul...the Voyager is not even a smartphone.
All of us Mogul/P4000/PPC6800/XV6800 users are in the same boat; ram is un-upgradeable and future revisions/updates to further utilize the currently installed ram to better efficiency is slim-to-none.
With this in mind you have (3) options:
1. Install a application with true-close functions for programs and minimize the number of apps running at one time...
2. Make a LATERAL (IMO) move to the HTC Touch or Dual...
3. Downgrade a bit to a multimedia device like the Voyager, etc...
Here is a link to a thread that has alot of imformation and commentary pertaining to 6800 ram...
http://forum.xda-developers.com/showthread.php?t=350002