Admin Options Change Using AJAX?

I’m writing a plugin to announce blog posts on Twitter. The plugin will give the user a choice of URL shortening services from the admin interface. The URL shortener is chosen from a drop down list. I’d like the display of the options page to change based on what is selected from the drop down so that fields for username and/or other authentication can be displayed if the selected service requires such information. I’d like this to be shown using AJAX but I don’t have the expertise (yet) to do this myself. Can anyone point me in the right direction so I can learn how?

Thanks,
Dave