Hmm, I don’t get a horizontal scroll bar at all. I don’t have a mac nor a horizontal scrolling wheel.
So I’m going to guess that you are looking at the page on a browser width of 1024px or less? In that case, I would say you should make your pages only 960px wide, since that is the standard width to use for 1024×768 resolutions – read a bit more about it at the 960 grid system site.
Better yet, set up your css using media queries. This will take a bit more work, but I really think it’s worth it in the end. Try narrowing your browser window while looking at this site, you’ll see things shift, change styles and rearrange.