i am using Ajax Autocomplete for Jquery ( http://www.devbridge.com/projects/autocomplete/jquery/ ) in one of my application. The Search Form looks something lik
Ajax Autocomplete for Jquery
Alternatively, you can specify the params using a function which is evaluated just before the ajax request is sent.
$('#q').autocomplete({ ... params: { 'entity_type': function() { return $('#top_search_select').val(); } } ... });