Ok.
Considering it is an old system, debian is a good one for this type of hardware.
Now that RedHat has changed a bit, introducing fedora, etc, I suggest to still dabble with red hat. Suse could actually be the least accessible to you unless you buy it. Familiarize with both red hat and suse as well as debian, however, keep in mind to learn say, linux system files and unix/linux command line, then it wouldn't really matter what distro you use as you'll be versatile in linux in general. Some distro have their own ways of doing things so being exposed to a couple of distros can help; knowing how to install them,etc.
Someone mentioned slackware. If you are a beginner, I say, no way jose, not for a beginner.. For that, start with debian.
Openbsd is nice. Have your entree with Linux first & venture out to others.
Glad to be here!