Make a note to think about this later: The simple code to call XMLHttpRequest in these examples is not
thread safe. If you want to update TWO parts of your page at the same time, things get trickier. This may not be an AJAX beginner topic, but I mention it now because eventually this will bite you and maybe you'll remember to Google for "xmlhttprequest threadsafe". I only learned this last week!
AJAX winds up with lots of script. Do you use FireFox, Dom Inspector and FireBug? Those are great tools to debug JavaScript execute.