How can I make B be a fixed 800px size while A and C fill the space, so they are all together touching? http://jsfiddle.net/8ufuV/
Fixed it up for you http://jsfiddle.net/8ufuV/17/ You need to float all the elements left; and then specific a width (don't forget about padding/margins when doing so). On an element you want to appear below them add clear:left; to it.
clear:left;
Good luck