Rails - Mar 2006 - using ajax-enabled search field in rails

Hi.

I followed the ajax-the-rails-way in the (quite usefull) Agile web
dev. with rails. I have a search-field where I can enter a postalcode
(zipcode).

<%= observe_field(:search,
                  :frequency => 0.9,
                  :update    => :results,
                  :url       => { :action => :search })  %>

Is it possible to have both the :frequency and another conditions
which starts searching when two or more digits have been entered into
the search-field? The db is postgresql and I use the wildcard ''select
* from postalcodes where postalcode like xyz%'' where xyz is the value
entered in search.

regards
Claus