Where to put 2nd domain controller?

I’m running 4 Win2k servers. The first is the brains of my operation, amoung many tasks, its the first domain controller. The second box is a web server that stands behind the DMZ port on my firewall. The thrid is my Exchange 2000 box and the 4th ismy primary file server. I’m think that the file server is the most logical place considering that the web and mail server don’t seem appropriate. Ideas? (And no, I don’t have another box to put the controller on.) Thanks, matt.