Slackware Linux is considered to be one of the "OG" Linux systems. It is incredibly stable as well as probably too fast for you. Even with hardware from the Age of Clacking Stones, you can have a server or workstation up and running in no time. Various server environments are included with Slackware, which is based on Unix, and are ready for deployment. Also included is a fully functioning and completely adaptable desktop enviornment.
Let it be stated that Slackware probably is not intended for the Linux Noobie. Yet if you are ready for a challenge, then start slacken! Dont worry, there is plenty of help.

Slackware Linux Homepage
Slackware Forum @ LinuxQuestions.org

Debian GNU/Linux or simply Debian is a stable and quick distro that is based on the GNU/GPL and the Linux kernel. Debian is maintained and updated by a group of people that offer their services gratis(that means for free). This system is secure, stable, and uses a fraction of your resources. Installations are pretty straight forward but configuration can be a little intimidating if you are new to the Linux structure.

Debian Linux Homepage
Debian Forum @ LinuxQuestions.org

Gentoo Linux is a user friendly distro. Besides having a Graphic User Interface (GUI) to assist in your installation and configuration, you still have many options to choose from. Portage, the powerhouse behind Gentoo Linux is a program for updating, distributing and installing software. Whether you are an experienced usesr or new to the world of Linux, this is one of the many good first's.

Gentoo Linux Homepage
Gentoo Forum