Project Server article: Extract Project Web Access site data to a new content database
Useful article that just got published on TechNet: Extract Project Web Access site data to a new content database
As mentioned in the article:
Having a content database used solely for your Project Web Access site data is a recommended best practice when deploying Project Server 2007. This creates a cleaner configuration and reduces the chance of problems when executing other operations such as backup and recovery, migrating data to a new farm, or migrating to a newer version of Project Server in the future.