<div id="wrapper">
    <div id="maincontent">...</div>
    <div id="sidebar">...</div>
</div>

#wrapper{
    margin:0 auto;
    width:600px;
}

#maincontent{
  border-right:solid 200px #DFDFDF;
  position:absolute;
  width:400px;
}

#sidebar{
    background:#DFDFDF;  
    margin-left:400px;
    position:absolute;
    width:200px;
}

http://woorkup.com/2009/10/11/really-simple-css-trick-for-equal-height-columns/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+Woork+%28Woork+Up+%7C+A+Fresh+Charge+of+Creativity%29

Spalten mit gleicher Höhe

css

CSS