Macromedia Another $$$$ to spend....
I can't see How you are going to store a data non-visible into a drop down menu.
Either your drop down list is really small and you can just run PHP to assign to correct category against the selection made;
<?php
if ( $selection == "Speakers 80 Watts" ) { $cat=1; }
?>
or your selection is hudge and I can't see why you are using only 1 drop down menu ??
Surely in your example a bit of javascript with 2 drop down menu will work best:
Drop_down menu 1: Select the Category (here you go)
Drop_down menu 2: Select your Item
So when you change the Category on drop_down menu 1, it auto changed the selection available in drop down menu 2.
example:
http://www.felgall.com/jstip22.htm