Hi there,
You're better off asking this in a dedicated CSS forum as you'll get more viewers and better replies.
http://p2p.wrox.com/css-445/
You may also want to search Google for something like "liquid design" which is a concept used to create sites that adapt to the screen width.
All in all, it depends on what you're designing or what for. Sometimes a fixed width makes sense, sometimes a liquid design.
Cheers
Imar