Bound comboboxes on form

I want to create two bound comboboxes for selecting cell operators within a range of available countries. I want it behave like this: Combo 1 will consist of list of available contries -> user selects a country and the second combo will requery the database and show the list od of all available ooperators in this contry. I've searched the phpclasses and found the class called "linked_select", but I'm having problems with it. So I'm thinking about embedding and iframe which will contain only the second combo and requerying will be done replacing a a source url when user selects a country from first combo using a javascript onChange event. Now here's my question. Is this doable? If yes, how do I get the value from the second combo into original form for submitting? When I create an iframe from within a from, will the value be submitted with the form or not? Any thoughts greatly welcome. tia, Dezider.