Please check http://birhayalinizinde.com/?p=92.
The width of main content is 640px. That area could be 640px width, may be a little bir wider. The width of sidebar is ok.
But as you can see, when narrowed the main content the 2 empty spaces on both sides of the iste will be larger. So we need to narrow the menu (header) zone, too.
I hope I could have explained it right.