fadein,fadeout

a.js

function show(html){
  var target=$("#targetID");
  var old=target.children();
  var next=$("<div>");
  next.html(html);
  next.css({position:'absolute',left:0,top:0,width:'100%',height:'100%',display:'none',background:'white'})
  target.append(next)
  next.fadeIn(1000,function(){old.remove()})
}

$('a').live('click',function(){
  var url=$(this).attr('href');
  $.get(url,function(val){
    show(val)
  })
  return false;
})