fresh_when/stale? usage

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

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

##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

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?

The stale & fresh blocks already will send not modified back to the client. Since you are also sending it, you get a double render.