I also want to chime in about the Celeron. I would never think about buying a server with a Celeron in it, these procs are aimed at lower-end desktops.
Also keep in mind that you should not throw hardware at something that is a software problem. Figure out what you need and get rid of anything you don't, there is no need to stress the server for useless stats and the like. In time you'll need more powerful hardware if your forum grows, but please keep in mind that new hardware is no replacement for proper code!
You'll also want to have a look over at vBulletin.com's forums, they can help you get your box tweaked.
http://www.vbulletin.com/forum/forumdisplay.php?f=14
This thread has been closed because of the flaming.