I ran accross a great article on setting up Ubuntu 5.04 for use as an ISP or web hosting server.  It goes through the setup process step by step, complete with screen shots. Here is the link to the origional article:

http://www.howtoforge.com/book/print/59

I have also created a PDF with the contents of the above article, just in case it goes away.

An interesting project that is mentioned in the article is ISPConfig. It is an open source software package for managing websites at an ISP level. Here is the url to the website:

http://www.ispconfig.org/