Though I don't use 64-bit machines myself (for the desktop use they still have too much trouble, non-working software/programs lacking 64-bit support, ...), I don't think you should (or if it's even possible) try to "upgrade it to 64-bit". Even if that was possible, I'm sure the downtime is much longer than if you just did a clean install. You have backups, or should if it's a server, but make as recent ones as you can, copy your server's configuration files to a safe place and do a fresh install. Or better yet, you should have an identical machine for testing (production machines should have a "clone" test machine, on which upgrades can be tested before hurting the main machine) and do the install on that first and see if it even helps. I don't recommend upgrading a working server from 32-bit OS to 64, unless it is really really really needed. Don't fix a working thing. What is it that doesn't work without a 64-bit OS?
Just a friendly advice, if you insist on upgrading the system, and especially if you insist on doing that without a complete clean reinstall (formatting etc.), you are going to trash the server and have a very long downtime fixing it.
EDIT: unless, of course, you're very good in it - but if you were, you probably wouldn't be asking. So I assume you haven't done it before, and therefore should try to stick with methods that work.
Last edited by b0uncer; 08-20-2007 at 12:36 PM.