My favorite is Debian. www.debian.org Although the install is a bit more involved. Mandrake is very good for beginners.
For a 200MHz machine, I would not recommend a graphical interface. You can boot straight to a command prompt really quickly and there are several command-line mp3 players.
If you really wanna see Linux at full potential, you can download something called a "live cd" and run it on your main (hopefully faster) PC. A live cd will boot linux from nothing but the cdrom, so no worrying about having to install to your harddisk.