expands in which way - if you are thinking of having the site cover the whole width of the screen, you need to have width="100%" (if in html) or width:100% (if CSS) in either you outmost table or div.
also you will probably need a background tile for your banner area that repeats so that the banner expands to cover the entire width of the screen.
Actually, if you're using semantic markup (meaning using the HTML elements in the manner they were originally meant to be used) along with CSS, you won't need to set a width on your outermost DIV container - if you even use one at all, since DIVs (like all other block-level elements) will expand to take up all the space that is available to them.