I think it would depend what it is exactly you would like to learn. For example if you would like to learn desktop functions of Linux, Mandrake, SUSE, or Debian may be the one to go with. If you are learning about running a web server, RedHat, Fedora, or CentOS would all be good choices. For the most part though, there are not any large differences in the major distros. It is almost like comparing Windows XP Home to Windows XP Pro .

I would put CentOS 3.4 on it (guiless). That is the best way to learn.

Well you could choose any distro, but I found CentOS really easy to install. Also, if you are into web design or game server hosting than you will most likely choose CentOS over other OS\'s, like Mandrake or Debian.
Fedora Core is also another good OS, but I personally think CentOS gets security issues patched quicker.
In my post I meant the best way to learn was to use a gui-less OS, since then you have to learn how to do everything through command line, and how the operating system works.

Every GNU/Linux distro will do anything the others will do, it\'s just a matter of how much configuration you want to do. On a PIII though, I would definately not choose a heavy DE (Gnome or KDE), or you will be disappointed with the performance in X. If it\'s going to be a server, you probably shouldn\'t be running X anyway - no need to.

I was impressed with Mepis also, and you are right - the Debian roots are a plus.

I have also switched all my home systems from SuSE to MEPIS. As a new user, it was very easy for me to test before switching (boot from CD), it found all my equipment (my printer uses an older driver but it works), and the support was wonderful.

Now they have a 64-bit version out and that is making my AMD-64 system screaming fast. Didn\'t think I was going to be making a change anytime soon but this was too good to pass up!