Fun with z-index :(

[Feznizzle]

Here is a link to my site:
http://0stage.justmedical***DOT***biz/

Yesterday I gave that site a tiled background, then applied a shadow effect. The site has space between the top two sections, header content and main page content. Because of this, a drop shadow below the header fell onto the main page.

To get it to look right, I applied z-index like this:

#headerWrapper {
position: relative;
z-index: -1;
}

#topmenubar {
z-index: 0;
} 

Now it looks the way I wanted it... but you can't click on any of the links in the header (Shopping Cart, About Us, etc).

Can somebody school me on z-index? How can I fix this?

Thanks!