observe_field to update render partial in a div

Hey all,
I'm trying to update a div everytime the value of my drop down box is
changed:
<tr>
<th width="25%"><label for="ci_citype" >Type</label></th>
<td width="25%">
  <%= select("ci", "citype", CiType.find(:all).collect{|p| [
p.name,p.id]})%>
</td>
</tr>

<div id="create_ci_container">
<%= render :partial=>'cipart/create_hardware_cis' %>
</div>

<%= observe_field("ci_citype",
   :update => "create_ci_container",
   :url => { :controller => :create_ci, :action => :create_ci },
   :method => "post",
   :with => "'citype_id='+value") %>

The above code is written in view create_ui.rhtml of ci_controller.

The content in create_ci_controller:

class CreateCiController < ApplicationController
layout nil
def create_ci
@ci_type_id = @params["citype_id"]
end
end

The content in create_ci.rhtml is:

<% if @ci_type_id == '2' %>
<%= render :partial=>'cipart/create_software_cis' %>
<% else %>
<%= render :partial=>'cipart/create_document_cis' %>
<% end %>

When I first load the page it renders cipart/create_hardware_cis in the
div create_ci_container. The problem is when I change the value of the
drop down box, the corresponding partial file is displayed and then the
entire page disappeared immediately.

Please help me to solve this.

Regards
Suneeta