<div class="center">
<div class="top"></div>
<div class="left"></div>
<div class="right"></div>
<div class="clear"></div>
<div>
css:
.center { margin:0 auto; width: 700px;}
.top{ background-color: red; height: 100px; width: 700px;}
.left { float: left; width: 330px; height:500px; background-color: blue;}
.right { float: left; width: 330px; height:500px; background-color: green;}
.clear { clear: both;}