With 900 computers at main office do you really want to move any 2 of 3 computers to a small branch office? Same goes with the moving the PDC role over to the branch office.
Read this article to learn more about UGMC
http://www.windowsnetworking.com/kbase/WindowsTips/Windows2003/AdminTips/ActiveDirectory/Whentouseandnotuseuniversalgroupmembershipcaching.html