Trouble with Creating Unlimited Area for Text

The problem I am having is with the site at this link:
http://www-personal.umich.edu/~davealee/jkim2/main.html

The problem is that I would like the content area (gray box in middle) to be extendable as far down as I would like. But the problem is that due to the background image directly behind it interferes when I extend the box beyond where it is now (You’ll see a discolored patch at the bottom of the box. When I extend the content area beyond the point, the image behind gets distorted).

I am relatively new to the development side of design and am doing this for a friend of a friend as a favor. Do I need to slice the website differently? If so, what do I do in Dreamweaver to accomplish the limitless content area?

*On a sidenote, what are some of the more free, easy to setup/use content management systems I could use on this website?