I think an efficient way to deal with this is to have an option (definitely server-based, possibly client-based as well) to limit the overall size of the OST based on its total size as well as percent free space on local drive. Then simply do first in first out. There should also be an option to limit OST usage based on age of items (e.g. only cache items that are 180 days old or newer). Then you can effectively manage OSTs, for example:
- Cache items based on age (e.g. 180 days)
- Limit size to 4GB (if that means you can keep only 37 days worth of emails, that's what you get)
- Limit size to no more than 50 percent free space (if you only have 3GB available, your OST is limited to 1.5GB)
I believe Symantec and EMC provide similar capabilities in their archive products with their client-side offline storage and Outlook Cached Mode should definitely do the same. Obviously improving overall performance within Outlook beyone 2GB is also a must as others have noted (2GB is way too small with UM and general corporate mail trends).