According to my books online
System and Sample Databases
mdf Mastlog.ldf model Model.mdf Modellog.ldf msdb Msdbdata.mdf Msdblog.ldf tempdb Tempdb.mdf Templog.ldf The system databases are master, model, msdb, and tempdb. Note: The default location of the database and log files is Program Files ...
Source: Installing SQL Server 2005
No mention of the resource database nor does it appear in my SQL Server Management Studio under System databases hence it is not a "real" system database.
What I did find about the resource "database"
The Resource database is a read-only database that contains all the system objects that are included with SQL Server 2005. SQL Server system objects, such as sys.objects, are physically persisted in the Resource database, but they logically appear in the sys schema of every database.
-- Mark D Powell --