If you are running in an environment where there is High Availability options like LCR or CCR or SCR step for the exchange mailbox then we need to make sure that you have at least Daily full backups on the Exchange 2007 Servers

Below are the some of the situation where you wanted to go for new Hardware

 Your hardware has failed

You are performing a dial-tone recovery, where Active Directory is intact

You are performing a site recovery, where Active Directory is intact

When migrating a Client Access server, Hub Transport server, Unified Messaging server, or Mailbox server to new hardware

 Steps to approach recovery of Exchange 2007 Mailbox Server

1. Search for the Crashed mailbox server from the active directory users and computer and right click on the same and click on Reset
2. Install same windows version on the new box with same service pack and hot fixes with same name of the Crashed Exchange 2007 mailbox server with the same IP address of the old machine. If IP address is changed then you may need to updated DNS
3. Allocate the storage on the new server which matches the older server
4. Install Exchange 2007 using command “Setup /M:RecoveryServer
5. Install all the hotfix and rollup updates
6. Create all the Required Storage groups and stores in the same naming standard
7. Restore all the Database stores with options “This Database can be over written by restore”
8. Mount the databases
9. You can create the new public store and check if the replication is configured properly and allow public folders to get replicated from other public folder stores
10. Then you may rebuild full text Index catalog. Index catalog helps to search for the documents and attachments in the messages. Index Catalog is for each Storage group
11. Document helps to rebuild Search Index

http://technet.microsoft.com/en-us/library/aa995966.aspx