Changing Opacity of multiple images with one click

Hi everyone,
Ive posted on the flash forum before but this is my first time looking for HTML help. Im creating an html site to complement a flash site which already exists.
Check it out here,

HERE

What i would like to do is include a function that changes the opacity of the icons on the main page. When you press BUILT at the top, the unbuilt project icons would dim 50% say, and when you press UNBUILT, the built project icons would dim… Just an easy way for people to sort this basic information.

Can anyone point me in the direction of some tutorials or functions that would accomplish this for me?

Thanks for any help