I'm looking for some basic information on setting up a hosting enviroment. We are an ISP, and we do Design, and Marketing for websites.

On the server side, would we install a seperate database and codebase for each customer, or would you install it once, and make seperate IIS Websites pointing to the same codebase, and rely on host headers and portal alias to display the correct content/feel?

I really haven't found much information about this type of setup. I would appreciate any advice or experiences you have in this arena. Thanks.

You could manage multiple sites via portals, aliases and a single database but separation of clients is not all that easy, and errors on one portal can affect all portals.

I personally would never host different client sites on the same DNN installation.  I do however use multi portal solutions for my own sites.