It looks from the code as if this was originally meant to be a left sidebar, so things are a bit cockeyed.
However, what I think is happening is that, within the #wrapper div you have a #casing div. The wrapper is 1000px wide, whilst the casing has no width specified, so it takes up the full width because the wrapper is its parent.
The right-floated sidebar then comes along and finds there’s no room next to the casing and so drops beneath it.
To fix this, I think the easiest thing is to output the sidebar before outputting the casing. The sidebar will then float right taking up the rightmost 330 + 20 pixels, and the casing content will flow around it in the normal way with floats.
It’s hard to say without seeing the code, but I guess this is simply a matter of relocating the call to get_sidebar() in whatever file(s) contain it. Probably, but not necessarily, index.php and any specialist files like category.php that are called for special purposes.
HTH
PAE