New Search Field

Hi,

I would liek to put a new SEARCH field onto my site..

When you JOIN there is a POSTCODE/ZIPCODE field as mandatory..

ON ADVANCED SEARCH I would like to have a DROPDOWN BOX called NEAREST CITY.. options like this
NSW - SYDNEY
NSW - BYRON BAY
QLD - BRISBANE
QLD - CAIRNS
when one is selected, the result will be anybody whos ZIPCODE is included in each selection ie '4545,4546,4547' behind the scenes..

How can I do this? any ideas..