I am just beginning to use Linux and its applications (Ubuntu 8.0) and want to add Amarok as an mp3 player.  I have read elsewhere that in order to play MP3 files in Linux, I must convert MP3 files to OGG format.  From what I read here and elsewhere, however, that isn't really true.

Must I convert all 5,000+ of my files from MP3 to OGG to play them with Amarok?

Swiete bozie!!!

No, you do not need to do this.
But you have to make sure that you have the necessary mp3 codecs installed as some distributions will not do this for you automatically. Refer to the Wiki (aka Infobase) on the Amarok Webpage in case mp3s do not work for you.

It was installing xine that got rid of the 'unable to  play mp3' messages for me.

I'm sure there are other applications/codec packs that would achieve the same result.