Exchange server 2003 secondary server setup help

We have more than 200 users. We are handling them with Exchange server 2003. We are planning for a secondary server that could support them in case if the first server gone down. We want our user to use and access their email and mailbox with the second server then.
We need some advice or some tutorial which can let us make sense to create such secondary backup server. Is there anyone using that system?
