Placing all content within a middle div?

Let’s say I make a page with this as the background:

Now I know it’s wide and all, and I can trim it down to whatever is a better bg size (1024? 800?), so let’s say I’ve done that.

What kind of CSS would I use to create one div that would be centered in that black middle column, where everything could be added within it from there on in?

Thanks so much in advance!