What sort of email do you use? If there's some chance that you can connect to it via IMAP, then perhaps you could save some/a lot of space by downloading only a partial collection of all the mail and leaving the rest online.
If you're just trying to have your mails stored on an external drive, I think you could rig a setup with Mail too - Mail stores all the mails and settings in ~/library/mail, so perhaps you could move the folder to another drive, then create an alias to it (but remember to connect the drive before launching mail).
I have not tried this myself, so proceed with caution! Please don't kill me if you lose your mail
That said, backing up your email is done this way too, so I don't think it's much trickier than Thunderbird.
You could probably try the same trick with iPhoto too - move the files off to another drive then alias them.
Part of the slowness you're experiencing is probably due to the stuffed hard drive. Been reading on the forum that as your drive gets fuller everything starts to lag. Perhaps you could look into upgrading the version of OS X too - there were some speed bumps along the way, though the 800MHz G4 may not support it.
I'm sorry you don't seem to be liking the Mac too much! I've used my PowerBook G4 for about 4 years now and each version of Mac OS X has made it all the more enjoyable. I certainly hope things get better for you
