Your CSS now shows a 1000px wide .page holding a 200px wide .sidebar and a 780px wide .maincontent.
See the box model here
. The box model says that whatever you put inside an element cannot be larger than that element. margin/padding/border all count when figuring width/height.
To hold another 200px wide .sidebar, your .page would have to be 1200px wide and your new .sidebar would need to be placed in the correct spot in your markup.
Have a look at a simple 3 column layout here
. Notice only the columns are floated in that example, that may or may not work for you.