If you just want to speed up mail and have enough hard disk space, just drag your archive mail folder from the Mail.app list of mailboxes to the desktop and you have a nicely archived mbox. Delete all the e-mail you just stored and you're done. If you ever want to go back to these messages, just redrag the mbox to the Mail.app list and everything is as it was.

This is just what the doctor ordered. Drag that .mbox file over into a folder that gets backed up regularly... sweet!

Glad to be of help. This is really the best way to do it. I have 2 new .mboxs for each year ("in" and "sent") and store them on a hard drive that is backed up. Anytime I need to see an email from last year, its a snap to
re-place the appropriate mbox in the mail.app list of boxes.

You can double click those mbox files with out moving them, and mail opens a new window for that file.