You are afraid of losing your MP3s, but what if by some bizarre chance you have a system failure? You will lose some data, so burning those MP3s to CDs or DVDs will be better in the long run.
I find reinstalling all my Apps to be a pain, so i keep them on a separate partition, only 3rd party Apps, all the others stay in the default folder.
Archive and install works fine, but if there are problems in files that are left behind... you will know when the system behaves poorly. I've done this a number of times, moves all the data to a folder in the root directory called "Previous System." Only odd problem i encountered was the problem with deleting that folder within X, sure I could in 9, but X will think the folder is missing and keep giving you incorrect length of some record problems.