I just installed Panther today and chose to do a archive install. Basically it preserves everything in your home user folder (bookmarks, Apps, Mail accounts, Address Book, Internet config ) and the previous system folder is replace with the newer one so things like third party drivers or other software that required to be placed in the root level (?) will be trashed. Its easier to re-install those few items than your entire home directory (was that clear enough?).
In my opinion a clean install isn't necessary since the system folder is completely replaced and is basically a new system. But little things are left behind (invisible items but they won't interfere with anything they are just data files or folders ) by apps in your home account that you may want to get ride of but having to reconfig everything and re-registering apps makes it for a none option.
I love how Panther reinvigorated my system (Blue&White smurf tower) and extended its life for another year but hopefully I won't have to wait that long until I can get my dual G5 3 Ghz

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