I found a few more good sources for custom XP themes (Visual Styles). The first is in the forum section of the Neowin.net community.  The Customizing Windows XP forum has a subsection labelled Completed Visual Styles.  This forum contains hundreds of posts and some great designs.

Deviantart.com is another great site that features a massive selection of artwork including, but not limited to, Visual Styles.  The Deviantart site is slick, easy to operate, and displays thumbnail views of each style in a well organized manner.  Selection is a snap.

Lastly, the aptly named XPThemes.com contains a large assortment of Windows XP customizations.  From cursors, to logon screens, to Visual Styles and Windows Blinds, to system tweaks, this is a one-stop site for customizing your Windows XP installation.