Look at this page (http://www.hawkwings.net/2006/06/02/three-pane-mailapp-hack-that-works/) then goto this download page (download the plugin) (http://harnly.net/downloads/MailWidescreen.html) and read the instructions! You now will have "widescreen" preview in Apple's Mail.

http://www.hawkwings.net/wp-content/uploads/2006/06/wp-images/threepanehack-tm.jpg