Does the content of the external template change depending on a users actions (ie an tick box is clicked or an option selected)?
Does the template need to be loaded multiple times into the same div (ie every few minutes or every time a button is clicked)?
If the answer to either of these questions is yes then you should be using ajax. Something along the lines of the following would work:
$.ajax({
url: '../mytemplate.html',
success:function(data){
$('.overbox').html(data);
}
});
Please note that this is simplified and should also error check etc. For more info see the jquery docs on ajax or 'The Perfect jQuery AJAX Request'.
If the answer to both the above questions is no then you should be using a server side language to include the file in the div before it is sent to the client.
An example of this using PHP would be:
<div class="overbox">
<?php include '../mytemplate.html';?>
</div>