I don't believe that you should need to set a width on the right div. By default a divs width is 100% and if you have a div with a fixed pixel width preceding it will cause an issue because the browser will determine the percentage width of 82% and if the pixel width doesn't fit inline with the content then the content will be pushed down.
There are a few other options, such as setting a minimum display width for both regions or setting your navigation to a percentage-based width. But that would really depend on the layout you are attempting to portray. If you have a picture of what you are trying to accomplish with your layout people here might be able to give you suggestions on how to best approach this.
my idea would be to try using min-width on your containers.
What I suppose is happening is that eventually, as the browser window reduces one container gets too small to hold it's content. Restricting the minimum width will stop that happening and I'm guessing that there comes a point at which the page will be so narrow it will look odd anyway?
Thanks for your suggestion. Here is a link to the index page of the site that I'm building:
What I'm trying to do is have the navigation in the main content div to span accross the rest of the content area. As you can see, when you contract the browser window then the entire right div drops.
CSS 2-column, liquid layout.