Exchange Enterprise vs, Standard? (2003)

I learned the hard way that Standard Exchange can only hold 16GB worth of information in it’s two main database files for the mailbox information store…and have now have a dilemma of either coming up with and enforcing limits on peoples mailboxes or upgrading to Enterprise that allows for Terabytes worth storage. I’ve investigated other ideas, like personal folders use and find that most don’t want to do that because information is not readily available from any machine…

Has anyone ever had to upgrade and started using server clusters? Is this transparent (I assume it is)to the user or is it slower with apps like webmail?