The pre-installed apps on the 5500 are located in ROM. Only the links to the files are stored in RAM and these take an insignificant amount of memory.
Some of the PIM apps are set to "fast load", which means that they are pre-loaded in RAM, ready to run. If you are willing to sacrifice load time to free up memory, tap & hold on the icons of the Datebook, Addressbook, and ToDo apps, then uncheck the FastLoad option.
Another alternative is to create a swap file. If you have a SD or CF card for storing apps, you can then free up some "internal storage" to be used for swap, which will help when running multiple apps or opening large files. Search this board or the Devnet board for info on creating a swap file.