Exchange vs. AD/domain controller

Hi
Here is my dilemma, I currently running on a Dell PowerEdge server several services,
Exchange, Active Directory/Domain controller, Shared files, Norton AV Corp.
We recently purchase another server to divide this services in.

I would like to reduce the number of services running on this server,
But I?m not quiet sure how to divide the services or/and what will be the easiest/less complicated
way of doing this. And what I mean is,
should I moved the Exchange to the new server???
leaving AD/Domain controller and NAV/Shared files in the old one
or should I moved AD/domain controller/NAV and Shared files to the new one???
what ever I moved over to the new server I have to consider all the changes I will
be force to do on my firewall, down time etc?.

Please advice.
Thanks