2 databases in sql server 2005 "Production Server"
Having a test database on a production instance sounds like a very bad idea to me, even without taking performance into account.

However, a second database on the instance does not consume resources per-se. It's the user or application activity that drives load to the instance. Obviously, you will need storage to accomodate the data and log files and that could degrade the NTFS performance if the free space on the volumes goes lower than 20%.

Other than that, buffer consumption and CPU/disk/network time depend on activity, not on the database existance alone.