If you absolutely have to distinguish between IE versions, you can use a "condition comment". This is an IE-specific feature which looks like this:
<!--[ie IE 6]>
....everything here will only be seen by IE6
<![endif]-->
In this example, all other browsers just see it as an HTML comment and ignore it. There is also a variation which allows other browsers to see it, which means you can also use it for things that you explicitly want to exclude only from particular IE versions.
Note that Conditional Comments are used in the HTML document, not the stylesheet, so you'll need to either embed your styles in the HTML, or have separate stylesheets for different browsers.
If you're coding for IE6 it has a habit of being unavoidable, but in general I'd prefer to avoid browser-specific code wherever possible.
It's better to use feature detection - in other words, your code looks for specific features that you want to support, and reacts accordingly depending on whether the feature is available or not on the browser. This means you can cope with unexpected situations such as a user with a browser you've never heard of, etc.
With that in mind, I'd recommend using Modernizr, which loads your site with CSS classes depending on the available features, which you can then use to help you decide how to present your site on any given browser.