How to theme one tab of a node I have hook_menu() configured to show a tab for every node at node/%node/summary.  The callback for that page returns the correct content, but I'd like to add a bunch of HTML to the page.

I believe I need to create a .tpl.php file in my custom theme.  How do I know what to name the file and make sure that the template is used only on the correct pages?
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.