As thread_killer said try out linux in that manner or go with one of the BSD's. I have started reading "Absolute OPENBSD Unix for the Practical Paranoid" by Michael W. Lucas $27.17 from amazon.com. Lucas does say that the book is for experienced Unix users, but if you have the basics of linux down, you shouldn't have too many problems working your way through the book. Right now, I am working on setting up an OpenBSD firewall box.
The box you have is more than enough for a home network firewall. If you want to setup a linux desktop, I would get more ram as nihil suggested, although you could run a liter desktop than kde/gnome for better performance.