Here's a couple sites I came across while searching for RPG Maker XP resources.
http://www.phylomortis.com (advanced rgss scripts for XP, tons of ripped sprites for the older rpg makers 2000,2003..none for XP however)
http://www.phanxgames.com/resources/index.php?cat=3 (this one seems to have alot for XP)
http://www.crankeye.com
http://www.angelfire.com/rpg2/rpgcommunity/Resources.html
http://www.rmxpu.net/dev/rmxp/
http://www.kobrasrealm.net/resources.php?rmxp