AJAX :: Adding "selected" To Dynamic Drop Down

I am in the process of adding a dynamic dropdown of cities that populates based on the county/state selected by the user.

I have managed to get it working but the problem I have is that when the user goes to edit their details this dropdown list does not:

1) Show the dynamically generated list of cities because the list uses the javascript onchange element i.e onChange="AjaxFunction(this.value);"

2)Have the city value that is stored in the database selected as default.

Here is the Ajax/Javascript used:

Code:
<script type="text/javascript">
function AjaxFunction(cat_id)
{
var httpxml;

[Code].....

The first problem is to get the list to dynamically populate on page load and the second is to include the users selected city as "selected" in the options created by javascipt and ajax functions.