At the moment i dont have IE 6 or 7 near so i cant say much. At the moment, your layout_body is 995px wide, while it contains 2 items.
One which is the main, which is also 995px wide, which is the same amount as the container, though u also have your side_nav next to it with a total width of 240px(padding included), so next to eachother that will in total give you 1235px while their container is only 995px wide..
So what u could try is setting the .main to the same width as the .content within it, which will be 720px and also give it a float.
I can't test this for myself so please someone correct me if im wrong