If I understand your objective correctly, you're trying to do something that is impossible and you'll need to let one of your requirements go.
Your site has two table cells next to each other. They grow and expand together even if the content in each is not the same length.
To centre your banner across the whole site, it needs to go underneath both columns. If your left column were longer that your right a site centred banner will get pushed away from the main content.
To keep it close to the main content, it needs to go into the right column, where it is at the moment. You can centre it that more restricted location, but it will look lopsided when you have other content of roughly the same length as you do currently.
As an aside, the banner is currently breaking your site by pushing the right column out further than it should (at least in Firefox, it may be different in other browsers), making it look even more lop-sided.
(as an aside, I'm guessing that maybe you work mainly in print? where you have absolute control over page layout in a way that isn't so for online work where the elements are often generated dynamically and you often don't know in advance exactly how much space they'll need.)