Using fadein and append

This should solve your problem I think.

$('#content').prepend('<p>Hello!</p>');
$('#content').children(':first').fadeOut().fadeIn();

If you are doing append instead then you have to use the :last selector instead.

I don't know if I fully understand the issue you're having, but something like this should work:

HTML:

<div id="posts">
  <span id="post1">Something here</span>
</div>

Javascript:

var counter=0;

$.get("http://www.something/dir",
    function(data){
        $('#posts').append('<span style="display:none" id="post' + counter + ">" + data + "</span>" ) ;
        $('#post' + counter).fadeIn();
        counter += 1;
    });

Basically you're wrapping each piece of the content (each "post") in a span, setting that span's display to none so it doesn't show up, and then fading it in.

If you hide the content before you append it and chain the fadeIn method to that, you should get the effect that you're looking for.

// Create the DOM elements
$(content)
// Sets the style of the elements to "display:none"
    .hide()
// Appends the hidden elements to the "posts" element
    .appendTo('#posts')
// Fades the new content into view
    .fadeIn();