<script type="text/javascript" src="http://jqueryjs.googlecode.com/files/jquery-1.3.2.min.js"></script>
<script type="text/javascript">
$(document).ready(function() {
  $.divliste = {
    '0' : $([]),
    '1div' : $('#1div'),
    '2div' : $('#1div, #2div'),
    '3div' : $('#3div')
  };

  $('#divsecici').change(function() {
    // hide all
    $.each($.divliste, function() { this.hide(); });
    // show current
    $.divliste[$(this).val()].show();
  });
});

</script>

<select id="divsecici">
   <option value="0">-- div seç --</option>       
   <option value="1div">1. div</option>
   <option value="2div">1. ile 2. divdiv</option>
   <option value="3div">3. div</option>
</select>

<div id="1div" style="display: none;">
1. div
</div>
<div id="2div" style="display: none;">
2. div
</div>
<div id="3div" style="display: none;">
3.div
</div>