Your best bet is to use "FAUX" columns. Google for it for more info.
To put it in a nutshell, design your graphic for the whole width (including borders/shadows) like:
I made up a demo some time ago.
See:
http://mandelbaumonline.net/css202/ for the
HTML
and:
http://mandelbaumonline.net/css202/202.css for the
CSS
However, on looking at your site, there are other compatibility issues you have to fix.
(Who or rather what wrote the code?)