I've been amusing myself lately in my spare time by seeing how various Linux Operating systems work on older machines. Slackware 13.37 runs on all of them without a problem, thus far followed by Red Hat, then CentOS, and then Ubuntu. I really like Slackware because it gives you total control over your system. You don't have to install the X Window System if you don't want to and the OS still works absolutely fine with it. Though, it is not for the faint of heart. If you aren't comfortable with Linux commands yet, my recommendation is Ubuntu and CentOS until you get to that point.