<!--[if IE 6]>
I am using IE6
<![endif]-->
That works.
How do I do "or" IE7?
If you, for whatever reason, what to only test for IE6 or IE7, and maybe have some other conditions for IE5 elsewhere, there is also support for other operators:
<!--[if (IE 6)|(IE 7)]>
This is IE 6 or IE 7!
<![endif]-->
Check out the wikipedia article which has better documentation than Quirksmode on this.