Search according to drop-down list

Hi, currently I have two separate search fields. One searches the
paginated model and the other searches a user model:

<% form_for :post, :method => 'get' do %>

<%= collection_select(:post, :id, @posts, :id, :title, options ={:prompt
=> "-Select categories-"}, :class=>"category_dropdown_list") %>

<%= text_field_tag :search, params[:search] %>
<%= submit_tag "Search", :name => nil %>
<% end %>

<input type="text" id="search_form" name="search" style="width:100%;" />
<%= observe_field 'search_form',
   :frequency => 0.5,
   :url => { :controller => 'auth_user', :action => 'get_results' },
   :loading => "Element.show('indicator_gif_id')",
   :complete => "Element.hide('indicator_gif_id')",
   :with => "'search_text=' + escape(value)" %>

I have two main questions:
1) How would I observe the collection_select and depending on what is
selected, run x or y action (automatically)
2) How would I merge the two inputs (I want one input that gets observed
to act as a 'live search')

Thanks!