How to theme one tab of a node
In Drupal 6, this would be:

 page-node-summary.tpl.php


More information on Drupal 6 template suggestions can be found at http://drupal.org/node/1089642

In Drupal 7, this would be:

 page--node--summary.tpl.php


More information on Drupal 7 template suggestions can be found at http://drupal.org/node/1089656

In either case, I'd grab the default page.tpl.php of your theme as a basis for it.