Umm... not to burden you further, but before you do any upgrade, you probably should test your main code against php 5. Make sure that everything works as expected. Things have changed in php 5 (for the better) and some things are disappearing in php 6 (register_globals, magic-quotes-runtime). So be careful. Many older php 4 scripts will down-right throw a fit because certain things are deprecated or not available ($HTTP_POST_VARS, etc.).
So take a test-box, and set up an exact mirror of your servers (a disk-image could help) and install php 5 using apt-get (to see how the install goes initially). Work out the upgrade kinks, then test your sites code. If something goes wrong, update the code using the test-box and lather-rinse-repeat. Keep going until you have your site the way it is now.
If you're hosting other sites (or student sites) make sure you send out notice to give them time to update their script(s) to php 5 code compliance. Otherwise you could be inundated with angry emails.
Of course you could install them concurrently and then use .htaccess to declare everything in a specific dir goes through the php 5 parser. Then copy the entire site into that directory and test from there. That's if there is no way to get an exact replica of your server. Of course, if you're dealing with a clustered set, you could use just one node as your "test-box" for php 5. Of course, if you're in IT, you should really have a test-box that you test all code (security updates, OS patches, program installs) on before you make it live.
Just some words of wisdom.
EDIT Oh, and it may be wise to offer an "over-lap" time if you're dealing with customer sites. Like say start offering php 5 at the beginning of next week (6 days from now) and then offer php 4 and php 5 together until say June of 2008. As of June 1, 2008 you'll be dropping php 4 and any scripts not updated will break horribly. Many hosts are doing this. Performance will take hit while running in php5 via CGI; however, many good things can come from it.
