side-by-side stretching divs

Hi,

I am trying to take the plunge on my first site without tables.

Here's my question:

I have a master container div, set to 420px high. Within that div, I have two divs, side-by-side. The left div is for navigation, and the right for content.

Now... sometimes, the content div will get stretched down because there is too much content to fit in the space allowed. However, when it stretches down, I need the navigation to stretch along with it (because there are corresponding background images in the navigation and content divs).

Did that make sense?

How do I do that?

Thanks for your help...

Tom