I agreed with David’s statements. Here at our company we upgraded to 2010 on both Project Server and SharePoint. On the project server upgrade, here are a few notes (lessons learned) which you may want to consider:
1) If you are going to upgrade project server to 2010 and having the "Project 2007 Compatability Mode" turn on, just remember once you do decide to turn it off make sure that a) all projects are saved/checked-in/published, b) PS server admin open and save the enterprise template and project server templates using Project Professional 2010.
If you failed to do so, you may have your projects files out of sync on the proper versions. It will be a mess to clean it up afterward.