If you’ve not bought the Agile Web D. book or the new Rails
Recipes book, you really should. A lot of the questions you’re asking
are in there along with a lot of really important concepts.
You need to look into observe_field which monitors a form field for
changes and then updates an element (usually a div with an id) with the
results from an AJAX call.
Then you need an action that builds the second select box based on what
the observe_field sends in.