I'm trying to create a page with a left sidebar that widens as needed, and a right content pane that expands to take up whatever space remains, and I'm trying to do it with two floating divs instead of table cells. Is this possible or do I have to settle for fixed percentages of page width?
This is possible, if you are just starting out using CSS then you will find that doing things and seeing what happens with each change is a good way to learn.
use: position: relative; float: left / right ; (depending on your layout) and than add a margin-top: xxx; margin-left: xxx; as needed.