I found 'code' (
http://homepage.powerup.com.au/~dagwood/web.htm#5) that
allows to strip attachments, which will definitely shrink the PST files
(and therby make them faster searchable), but I'm guessing that the
attachments are very valuable, and need to stay with the sender...
Since the code I mentioned Msgboxes each file, I'm sure you probably
would want to eliminate that line while parsing a (BACKUP OF A) 1.5GB
PST...
Maybe others have better ideas. Mine are just ...
1) strip all attachments. (the above mentioned method would output all
the attachments in a given folder, but may overwrite same filenames!)
2) age/Archive the .PSTs to split into more files, but slowly: 2 years
ago, one year ago...
3) export the emails to comma delimited (or Excel) and use an Access
database to search the emails in an imported table. Yes, this isn't
real time, No, it won't include Attachments, but at point X, when the
conversion is made, all the PSTs would be in a searchable database, and
then a new .PST could be made from today (or 90 days ago). Yes, it
means two databases, but new habits can be formed by archiving large
message attachments as they come in

. It's more secure (and
efficient) to use a file system (NTFS/FAT32) than a flat file format
(PST) for attachments, anyway, don't you think?
I don't know if this helps at all. I'm sorry. More info?