You need to keep an eye on your content models. When you float an element across, its containing block element effectively ignores that child’s height when calculating its own height (or width). This is something you are depending on with your header (which overflows horizontally).
Personally, I don’t understand why you are floating your sidebar across. Do you really want content to move under it if the content is longer vertically than the sidebar? Use a basic grid and decide either on % widths of your components, or on fixed sizes (responsive vs fixed layout). Chris has a good article in his blogroll on exactly that topic at the moment.