Syncing the saved mail ("On My Computer") will most likely end up causing problems. So, two options:
1. Hassle-free
Set up both computers for your IMAP account. IMAP mail stays on the server, so at least all your most recent mail will be available from either computer. When your IMAP account is full, archive mail to the Powermac.
2. Kinda tricky
Set up the Powermac as an IMAP server, using Postfix Enabler. Set up both your "real" IMAP account, and the Powermac account in Mail on each computer. Archive your mail not by saving to "On My Computer" folders, but to the Powermac IMAP account. That way you can access the archived mail from either machine.