Look for plugins in wordpress to do this or else it is going to be very complicated to this in code. TRUST ME
This link should get you going in the right direction: http://net.tutsplus.com/tutorials/h...o-build-a-kick-butt-css3-mega-drop-down-menu/ It teaches you how to build what is called a MegaMenu. The demonstration shows several different options and methods, and gives a fairly decent explanation of how and why it works.
My question would be that given it would appear to require a rather large fixed width, why would you want to piss on your site's accessibility with that garbage?
You can also use Google and search for css menu examples as they'll provide many sites showing you how it's done.