First of all, sexy drawing.
Second of all, I have two sites with a simliar concept as far as images at the top and bottom of the div, and i always seem to run into the same problem you are each time.
First of all, try to make sure you set a doctype, so that your browser isn't haphazardly rendering everything however it wants. I think there is more about that in the FAQ if you aren't familar.
Second, if you're going to use an image on one part, you're better off using an image on both parts. make a GIF image that has your background and the 3 px border, set the CSS to
and do the same for another div you have just below that one. You'll probably have to make an image for the top, too, but that will keep the feel of it consistant by doing that. Yes, it adds 3 images overall to your site... but the GIFs are usually 1K or so.