[cam]

Lets say you are updating your website completely, changing the names of page's filenames (ex: file.html to file.php) and so on, this is great, you should stay updated! But what if you want to get rid of those old pages without having to worry about those who go to the old web page and see nothing? It doesnt end there either, other visitors do include major search engines such as MSN, Google and Yahoo! If people are finding your old pages when querying in these search engines, and they attempt to go to that page that has been deleted or moved, they will get a "404 File Not Found" Error! Now i know you dont want that, no webmaster wants that!

Good article on using proper (HTTP 301) redirects;

http://www.stevenhargrove.com/redirect-web-pages/