Well, if WP would make it absolutely clear PRE-installation that in-depth theme customization and/or Theme Development is best left to the savvy front-end web developers or, at the very least, to the people who have -some- sort of front-end knowledge, then maybe there wouldn't be so many people flocking to the forums with the 'Help me theme my WP because I don't understand CSS or XHTML!' issues?
Just a thought?
The impression I'm getting is that everyone ELSE is getting the impression that WP is easy-peasy to custom theme by oneself when it isn't easy-peasy without having at least some working knowledge of what goes into WP theming.
For the record, I like WP how it is and have no complaints myself, but I actually do front-end stuff.
To the OP serenalove:
Yes, you will need to rewrite the CSS code, but in addition, depending on what you want in your second sidebar, you might need to rewrite the PHP coding if you want widgets in that sidebar and you will need to rewrite XHTML as well. If you don't understand PHP or CSS, doing this on your own will probably be rather difficult to do
For these reasons, I also suggest taking a look at some 3 column layouts, picking one that is closest to how you want your site to work, and then working from there to make it look like how you want it to.