How do I launch a thickbox with a submit form?

I have a search form:

<!-- Site Search START -->
<form name="form" action="/search.php" method="get"  target="_self" >
  <input type="text" name="q" id="searchform" value="Search" />
<!-- Site Search END -->

The form currently loads in the same window as my main site. I would love it if I could submit search.php?q=Search to a Thickbox search.php?q=Search&TB_iframe=true&height=480&width=640.