It's quite easy really. If you have already registered at
http://www.register.com, log into your account, then assign your web server's IP address to your domain name. This way, when somebody types in your domain name, it gets redirected to the IP of your server (Note: you must have a static IP from your ISP).
Your P2 system should be more than adequate to run as a web server, although with RAM prices the way they are, dump in at least another 64Mb.