rails fresh_when/stale? usage

Question

In my post_index action, I generate different kinds of "@posts" like..

def index
     case params[:listing_type]
          when "all"
                 @posts = get_all_post_from_memcached
          when "most_popular"
                 @posts = get_all_most_popular_from_memcached

          respond_to do |format|
               format.html
               format.js #for ajax reqeusts
               format.xml #for rss etc
end

##updated

    def index
         case params[:listing_type]
              when "all"
                    #the key here is teh same key I used for memcached   
                    if stale?(:etag => 'all_posts_key')
                     @posts = get_all_post_from_memcached
                     else
                      head :not_modified and return
                     end
              when "most_popular"
                     if stale?(:etag => 'most_popular_key')
                     @posts = get_all_most_popular_from_memcached
                     else
                     head :notified and return
                    end
              respond_to do |format|
                   format.html
                   format.js #for ajax reqeusts
                   format.xml #for rss etc
    end

From what I understand fresh_when takes a etag, and it is to be used if there is no difference in different kinds of rendered (in my case the rendering is different based on html or ajax)

and

stale? also takes an etag and surrounds the respond_to block.

In this case the etag will be different based on the different listing types. But it seems there isn't much flexibility in the way fresh_when or stale? can be used here?

Thanks

update. I changed the original block a little and now get a double render error what am I doing wrong, should "head :notified and return" just return the header and not touch the respond_to block?