scrivi i tag in minuscolo, usa un DOCTYPE appropriato:


*(margin:0;padding:0
body{
text-align:center;
}
#contenitore{
width:750px;
margin: 0 auto;
}
#barra {
width:750px;
height:164px;
}

#menu {
width:152px;
background-color:#1F1F1F;
float: left;
}

#content {
float: left;
width:598px;
}

XHTML:

<body>
<div id="contenitore">
<div id="barra"></div>
<div id="menu"></div>
<div id="content"></div>
</div>
</body></html>