.main{
width:1000px;
height:auto;
	
}

.box{
width:300px;
height:200px;
border:1px solid #000;
background:#ccc;
float:left;	
}

.clr{
clear:both;
}

<!--
overflow:hidden;
margin:0 auto;
-->

#layout { width:900px; background:#eee; margin:0 auto; overflow:hidden; } 

#main { width:600px; height:500px; border:1px solid #ccc; float:left; } 

#sidebar { width:270px; height:500px; background:#999; float:right; }