You don't have to use fixed width - if you would like a fluid site, you can simply set a container with either fixed pixel margins (so it strectches with screen), percentage margins or percentage width and auto side margins (to centre it). Inside this, a fixed width navigation bar can be floated, and a content div set to fill remaining space. As the container is fluid (either directly or proportionally) with screen width, the content will also be.
An example of this can be found here. Although this demo uses a fixed width wrapper, a fluid wrapper will work the same way.