how to update ranking in sortable elements?

hey all,
I have a sortable list like that:
# view
<ul id="list">
  <% 6.times do |i| -%>
<span class="item_position" id="item_position_<%= i+1 %>">
            <%= i+1 %></span> <li id="item_<%= i+1 %>">I'm number <%=
i+1 %></li>
  <% end -%>

<p id="list-info"></p>

<%= sortable_element 'list',
      :update => 'list-info',
      :complete => visual_effect(:highlight, 'list'),
      :url => { :action => "order" } %>

# controller
def order
    @order = params[:list]
    render :partial => 'list'

I would like to update the position number in the span item_position
as I drag and drop elements. As an exemple, if I drag element number 5
to the second position I would like to update its span to 2.

Any idea how to do that?

thanx in advance