SBS 2008 - Exchange Server 2007 Performance Issues

One of my customers has SBS  2008 Premium running on Dell T110 poweredge with 4GB Ram. The system is now nearly 4 years old and over time the users have been complaining about Outlook performance. Typically a message is shown stating that Outlook is retrieving data from Exchange Server and sometimes Outlook shows as Not Responding for periods of time before resuming normal operation. There are 8 main users and they all insist on sharing ALL of each other's folders including deleted, sent items, calendars etc. The users are disinclined to archive anything so on average the folders contain 2000 - 8000 mail items. I have experimented with running with the cache and without it but have had some synchronisation problems when the cache is on possibly because the offline folder becomes very large. The network appears to be operating OK and browsing files and accessing files on the server is quite quick.

My feeling is that the performance issues may be because of the combination of the mail box sizes and that every user is sharing every other user's folders. However I am not sure of this and would like to be able to find a way to be completely sure so I am not overlooking some other problem that could be causing these performance issues.  

Any advice on how I should proceed to solve this problem would be appreciated.

John