If you are not averse to adding jQuery to your site, the code is very simple:
suppose you have 3 buttons:
<input type="button" value="Item1" name="Item1" id="Item1" onclick="btnClick(this)"/><br/>
<input type="button" value="Item2" name="Item2" id="Item2" onclick="btnClick(this)"/><br/>
<input type="button" value="Item3" name="Item3" id="Item3" onclick="btnClick(this)"/><br/>
And the dynamic content is contained in HTML files (or ASP, PHP, etc.) named "Item1.html", "Item2.html", and "Item3.html"
And let's presume the dynamic area of your page is:
Your AJAX function in jQuery is as simple as this:
$('#dynamicContent').load(theButton.id + '.html')
This line of code:
theButton.id + '.html'
will be "theButton.id" which will be any of 'Item1', 'Item2', or 'Item3', concatenated with '.html', so you get "Item1.html", etc.
.load(), tells the page to load the page
$('#DynamicContent') tells it where to load it.
So you get
"set the contents of the DIV 'dynamicContent' to whatever is in the HTML page named the same as the ID of the button I just clicked".
Of course, you can rejigger this many different ways, and you don't need to use jQuery either... but the above should be enough of a working sample that you could use it to parameterize your AJAX call if you wish to do so.