To use a Read Only Domain Controller, you need to already have a Windows 2008 Domain Controller in your Domain. You will also need to prep the domain and forest prior to installing a windows 2008 domain controller.
For DHCP, you just have to mimic what you have setup on your current windows 2003 domain controller.
1. Install the DHCP service
2. Setup and activate your scope
If your users are in your current domain, you will not need to worry about them. But you can setup the Read Only Domain Controller to cache the user profiles.
For data, just copy them to your new server and setup file shares.