Greetings,
There are two ways to do this, but since I don't really use 10.5.x I can't be sure of one of them. In Mail, there should be an option to Archive Mailbox somewhere, but I just don't know because I don't use it. The other way is just to copy the folder of the mailbox you're looking to archive onto your Desktop or wherever you want to save it.
The drawback is that if you want to open these messages again, you can't do it from Mail; you would have to double-click on each one to open it, and the message name doesn't tell you anything about who it's from or what it's about. There's at least one third-party mail archiving solution on the market, but I don't remember the name offhand.
The best option is to always have a current, bootable backup of your entire drive so you can restore your Mac to the way it was before the hard drive failed or that unexpected power outage causes a bunch of files to get corrupted or just disappear.