Well, firstly i don't reccommend that you make the body 850px wide. I suggest that you create an id or class called 'Wrapper' or 'Container' and make that 850px wide otherwise, the footer will only stretch the 100% of the body which is 850px wide.

Once you've done that, don't put the footer in the wrapper/container, just on its own with the 100% width.

It's pretty hard without any code. Hope that helps!