I just found one of the most beautiful Group Policies that I've ever come across:
How to use Group Policy to allow the users to chose any screensaver except (None)
This post is from Group Policy Central, and is 4 years old, but I've verified that it works properly with Windows 7 and 8, and is just a beautifully done Group Policy. Thanks Kevin for creating it and thank Alan for sharing.
The below is excerpts from the posting:
Step 1. Edit a Group Policy Object (GPO) that is targeted to the users accounts you wan to apply this policy
Step 2. Navigate to User Configuration > Preferences > Windows Settings > Registry then from the menu click on Action > New > Registry Item
Step 5. Click on “New Item” then the “Registry Match” option.