I just posting to say this is pretty cool.
I'm running a server from my G5 tower now.
Disabling the default personal file sharing (http) and ftp access through the system properties is the first step, to avoid conflicts.
Then install MAMP from www.mamp.info -- easy, just install and it's done. Comes with working PHP, apache, etc. I haven't tried MySQL yet, though. But it's there and appears to be working. PHP is certainly working, with some scripts that didn't work on other computers, even.
For FTP, use PureFTPd for OSX from here: http://jeanmatthieu.free.fr/pureftpd/
It's also easy to setup. Then create an account for the FTP for your site and set the root for that to the htdocs folder within MAMP in the applications folder (as per the default configuration of MAMP).
(One of the FTP accounts I setup works great; the other works, but isn't allowing modifcation of the directory, including upload. It's a permissions issue, that I'm going to look into soon.)
And that's it.
I also had to forward ports 80 (http) and 21 (ftp) to my computer, since I have several computers attached to the same router. This is easy enough with my linksys router, though.