How can I update the DOM after Ajax Call (jQuery)?

I'd use $.ajax() instead of $.post():

$.ajax({
    type:'post',
    url: 'posts_in.php',
    data: $("#postentry").serialize(),
    success:function(data){
        var id='post349';
        $('<div></div>').attr('id',id).addClass('myclass').html(data).appendTo('#records_wrapper');
    }
});

Quick demo of how the div will be appended and populated: http://jsfiddle.net/AlienWebguy/zuPA2/1/