Content - минимальная ширина, равна 510px для IE7(8beta2}, OPERA, FF, Chrome, SafariWin
<body>
<div class="header">Header</div>
<div class="middle">
<div class="content">Content</div>
<div class="left">Left</div>
<div class="right">Right</div>
</div>
</body>
* {margin:0;padding:0;}
html,body {background-color:#aaa;height:100%;}
.header {
background-color:#9c9;position:relative;z-index:2;
margin:0 auto;min-width:910px;max-width:960px;height:100px;}
.middle {
margin:-100px auto;position:relative;min-width:510px;
max-width:560px;min-height:100%;}
.middle:after {content: "";clear:both;display:block;}
*html .middle {width:560px;height:100%;}
*html .header {width:960px;}
.middle,.content {
background-color:#fff;
border-right:#cc9 200px solid;border-left:#a99 200px solid;}
.content,.left,.right {margin-top:100px;overflow:hidden;}
.content {position:absolute;left:-200px;margin-right:-100%;width:100%;}
.left,.right {position:relative;width:200px;}
.left {float:left;left:-200px;}
.right {float:right;right:-200px;}