[D Brugge]

To expand on what Ashley has said, Yes, 1024 is the most commonly used screen width, but browsers need some of that width for their own "chrome," in the way of a vertical scroll slider.

Also, as wide screens gain popularity, many windows user are opting to put their menu bar on the side instead of the top. This also takes up some of that 1024 real-estate. That is why 950 - 980 pixel wide screens are common, to allow room for the "chrome"

Another option instead of fixed width is to use max-width and min-width. These css properties have not been supported by Internet Explorer in the past, but are supported in IE7 and IE 8. These properties do just as you would expect. They allow you to have flexible layout but with cut-off points. You can say "anything narrower than (min-width: xx px ) will appear as a fixed width with a horizontal scroll bar, and anything wider than (max-width: xx px) will appear as a fixed width with your background filling out the extra space.