[support] How to use ajax in form_submit_handler

[Scott]

I'm venturing into new territory regarding drupal module development.
I'm working on a module that creates a form for collecting information
used to build a url.  The url is for an api that returns XML (not JSON).
I need to know the preferred method for using ajax to either populate a
div on the existing page or redirect to a new page.  About all I've come
up with while searching is AHAH and JSON.  

I've done such things by writing my own ajax script and straight php but
never with drupal.  Help with best practices would be greatly
appreciated.