Andy Clarke has the answer in his Universal IE6 CSS project. I recommend the article on his 'For a beautiful web' site, as he has a specific approach to just this problem - which may or may not be for the same reasons you wish to carry out this ie6 only favouritism!
From his Universal IE6 CSS code:
To hide all your other stylesheets from Internet Explorer 6, wrap them inside this Conditional Comment.
<!--[if ! lte IE 6]><!-->
/* Stylesheets for browsers other than Internet Explorer 6 */
<!--<![endif]-->
Then add the Universal Internet Explorer 6 stylesheet.
<!--[if lte IE 6]>
<link rel="stylesheet" href="http://universal-ie6-css.googlecode.com/files/ie6.1.1.css" media="screen, projection">
<![endif]-->
See below for the full article, and the links within it for the full code:
http://forabeautifulweb.com/blog/about/universal_internet_explorer_6_css