Fix Background?

The problem Im running into is that I must have my site with a 850px fixed layout yet I want a black "header" bar to span the entire width of the page. I've been able to use the custom image background feature to give the illusion of a solid black bar that spans the page. Only to find that once I set it up to align with my header, to give it that seamless look, it's changes depending on the browser it is in as well as screen resolution. My goal is a black bar that starts at the top of the page and stops at the bottom of the "Horizontal Bar 2: Styling" . Yet stays fixed no matter the browser scenario to maintain a unified look across the page.(www.hercart.com).
Any help would be greatly appreciated.