You first need to figure out why your information store is growing so rapidly. Have you check mailbox sizes in System Manager? Keep in mind that to do a proper defrag on the information store you will need to take it offline. Online defrag will not change the size of the database.
Info available here: http://support.microsoft.com/kb/328804
you should do a defragmentation on the exchange db. everything the user does on his mailbox is registered there (deletion, addition) until you do the defrag.
and edit the exchange registry to allow larger db (75 GB) on exchange standard.
checkout this web page: http://technet.microsoft.com/en-us/library/aa998066.aspx
good luck friend.