CSS Layering? If you want to center a site a simple trick that I use sometimes is this div centering trick:
What this does is create a box (800 by 600 pixels) and postion the top left corner in the middle of your screen (or 50% to the right and 50% to the bottom) then we make the margins negative half the amount, thus postioning dead center. The postion: bosolute means that it still exactly 800 by 600 and is needed to make the hack/trick work.
The fallbacks to this is if the broser window is smaller then 800 by 600, the site will no logner be centered.
I hope this helps! But yeah, to reiterate the revious fella, its always good to know the basics!