I'm not quite sure why you want to set px width for your css style sheet. (I work for blind services in florida and am trying to redo our site using css because of the power it has to make sites accessible. % width helps with this be layout tables or css).
However, I too am working on a three column layout for "front page". I've learned a lot from
http://css-discuss.incutio.com/?page=ThreeColumnLayouts The page not only provides good information regarding different bowers issues but have some great links to css style sheets for 3 column layout.
Good luck