Don't Bother Upgrading to Panther
Now that the iBook has gone G4 I would suggest you get one. I cannot imagine running a Macintosh on anything but OS X.
However, if your machine does everything that you want it to do then there is no need to upgrade.
I think a new Mac would be a huge leap for you - think of the amount of iApp software that would open up to you.
If I was in your situation I would not bother upgrading to Panther.