Render content from page.tpl.php in ajax callback, without the html.tpl.php
There is AJAXify Pages (dev version only)


  Module to ajaxify site browsing and navigation. For instance, it
  allows for continuous audio playback via Background Audio module while
  changing the content of the pages.


or AJAXify module (D5 only)


  You can always create links that load other pages via AJAX but you
  will still load a full page HTML while you may only need a single HTML
  element from the loaded page and this is what AJAXify focuses on.


If youre not planning to use any of these module take a look at THIS function
to get some idea.