You have an even bigger problem looming. Outllook (up to at least 2003, don't know about 2007) has a 2Gb limit for its .pst file. Stops working after that.
I "solved" the problem for several users by making a copy of the .pst, naming it Archive 2003-2008.pst, then deleting everything in the current .pst pre-2009.
AND (very important) compacting the .pst file, to purge the deleted items. Don't miss this step.
Outlook allows you to open secondary .pst files as well as the current one, and gets around the 2Gb barrier.
However, this is just a hack to get them out of trouble. It does not solve the problem of users not filing important documents....and deleting the emails.
On a 100MHz network, a 2Gb .pst file takes a long time to open, which gets the users complaining about a slow network.
As a general observation based on a small sample, most of the bulk of .pst files is often photos, multiple copies of old documents, round-robin thingies (the pass-this-on-to-ten-friends things with pictures and music) and so on.
Good luck with solving your problem - and the user education program you need to run.
Ian