If you want the div centered just horizontally then use either..
or if you don't want to use absolute positioning..
margin: -(half height)px 0 0 -(half width)px;
That's the easiest way to do it horizontally. However, if you want it vertically also, then the link Templarian posted is the way to go.
margin: 0 auto;
Quick explanation of the link to get it vertically centered:
You are going to need to nest your #box inside of another div that is positioned at "top: 50%" and then add negative margin-top to your #box (half the height to be exact).
Hope that helps a little if there was anything fuzzy!