I suppose it depends on how much time that you want to put into it. You could simply stick an uppdate panel in the content area, using Conditional updates. The menu items would be the triggers. This method would require an ajax postback on each menu click.
Have a read here: http://www.asp.net/Learn/ajax/tutorial-02-cs.aspx.
Another option is to use hidden divs. jQuery is bit on the heavy side to do something so simple.
<script langauge="javascript">
function ShowContent(divID, otherDivID)
{
var myDiv = document.getElementById(divID);
var otherDiv = document.getElementById(otherDivID);
myDiv.style.display = 'block';
otherDiv.style.display = 'none';
}
</script>
<a href="#" onclick="ShowContent('div1', 'div2');">Number 1</a> | <a href="#" onclick="ShowContent('div2', 'div1');">Number 2</a>
<div id="div1">This is content 1</div>
<div id="div2">This is content 2</div>