I strongly recommend reading To Hell with Bad Browsers and A Web Designer's Journey. This two-part series at A List Apart discusses how they redesigned their web site to take full advantage of CSS (entirely eliminating layout tables), while still keeping the site usable by those with old browsers.

Here's an excerpt:

IF THE DESIGN OF THIS WEBSITE looks relatively coherent, congratulations! Your browser does a good-to-excellent job of supporting web standards like CSS-1, HTML 4.01, and scripting languages.

If this site is readable, but looks as plain as an Amish coat, your browser does not support web standards. Fortunately, you can easily upgrade to one that does.

Before you start shrieking, perhaps you'll hear us out.

Note that old browsers can still display the content, just in a plain fashion. This completely eliminates the need to maintain a separate version of the web pages (a sure recipe for disaster).

Wally Hartshorn