It depends on whether you want the banners to be in the flow of the document or not. You could for instance just place them absolutely into position (using #wrapper as the base starting point) following the format you used for some of your shadows that you have absolutely placed into position.
Alternatively if you want the banners to be part of the flow then create a new page wrapper wide enough to hold the existing wrapper content (980px) plus the two banner widths. Just give it a width and use margin:auto to centre it and then float three columns inside it (including the existing column).
The left banner column would be floated left with the appropriate width followed by the original wrapper which is now also floated left and then finally the right banner is floated left.