The forum uses less than 1GB, but obviously the more the better. We have 10GB of space here on EuroVPS, but that has to include the whole RedHat installation, the swap space and the forum.
Bandwidth on average we use under 3GB a month with compression, but we have used a lot higher than that on previous hosts. I think our record is easily above 10GB in a single month.
Latest versions of all are always good. Apache web server is a must, PHP 5.x and MySQL 4.1.x or better. As mentioned above Plesk is better than CPanel.
Shared / Personal hosting is a no. At least some kind of Virtual Server so I can configure our set up independently of other customers which means SSH and root access and the ability to remotely restart the server if I need to. There are lots of other things to take into consideration as well, but those are the major points.
And no, Frontpage is not required.
EDITED: 1 Aug 2007 12:44 by MATT