You can do it either with PHP and JavaScript by detecting the browser resolution and then loading a different CSS file. PHP is probably the best option as you may experience flickering depending on how you write your JS.
If you search Google for PHP/JS browser resolution detection, you'll get a ton of stuff. Otherwise I can find something for you when I'm at work tomorrow.
Alternatively you could design your site to have a liquid layout.
http://www.maxdesign.com.au/presentation/liquid/Hope that helps!