<!--[if lte IE 6]>
<p>You need to upgrade your browser</p>
<![endif]-->
This targets anyone with IE6 or older (lte is for "less than or equal"). If you only want to target IE6, just leave out the "lte" altogether.
IE6 users will still see the rest of the page. To avoid that you can put another conditional comment around the rest:
<!--[if (gt IE6) | (!IE)]>
<p>Page main content</p>
<![endif]-->
I haven’t tested this second part and I wouldn’t count on it working in all browsers. Most of us still use specific code to make our pages work properly in IE6 because many of the people using it haven’t a clue about how to upgrade and some (e.g. students in computer labs) don’t have the authority to upgrade.