Optimizing GPO : http://technet.microsoft.com/en-us/magazine/2008.01.gpperf.aspx

Understanding GPO replications: http://www.windowsnetworking.com/articles-tutorials/common/Understanding-Group-Policy-Replication.html

Under the Hood : http://www.windowsnetworking.com/articles-tutorials/netgeneral/Under-Hood-Group-Policy.html