Upgrading Win2003 Server to Win2008 Server

I would like to solicit advice, tips, and council regarding the upgrade of one of our servers from Windows Server 2003 R2 to Windows Server 2008 R2. This particular server is our primary domain controller, primary DNS server, DHCP server, and does some file-sharing. Of course, I have already started my own research at home and on Microsoft TechNet website. But, fellow IT professionals always have a different perspective and usually point out other things that MS doesn?t always mention. Considering that this is our most important server, I want to make sure that I do this right, and don?t ruin anything. I have already installed another Windows 2008 Server, which is acting as a secondary domain controller. But being that I haven?t yet upgraded the primary server, a few errors are occurring, which I believe will be resolved when I have completed the upgrade. The primary objectives for upgrading this main server are to take advantage of the increased security options, improved and expanded group policies, and of course, the upcoming release of Windows 7. It is my wish to upgrade our desktop clients to Windows 7 next summer, but before I do so, the servers need to be upgraded. Then I can move to the next stage of my research and testing. I would greatly appreciate whatever thoughts and expertise that anyone can lend.