You should check out Responsive layout.
Nowadays with both desktop and mobile devices consuming websites, there's a need to code for different environments and display resolutions. Read more there:
http://maki.bo.lt/responsive-layout
If you are designing for a fixed width, you're doing it wrong. Make your design scale to the browser window.
Um... I'm on a 27 inch monitor so I disagree with your suggestion.
Mobile devices are handled using the viewport meta tag.
Same width as the user's browser window.
...with the caveat that sometimes that's too wide: use max-width in your css.