I have always wondered how to get an extra column into wp. I normally just call the sidebar and footer from index.php as usual and then if I want a three column layout, put the two columns in the sidebar. e.g.

[/div][!-- end of id content--]
[/div][!--end of id wrapper--]
[!--index.php end--]

[!--include sidebar--]

In the sidebar.php I have two divs, one called leftcol and float this left and the second say rightcol and float it right. (It is a little bit more complicated than that but for ease this is how I will leave it).