come già detto, devi usare il float.

#barradx {
width: 156px;
padding: 0px 0px 5px 10px;
background-color:#EEEEEE;
font-family:Arial, Helvetica, sans-serif;
float:left;
}

#partecentrale{
padding: 0px 0px 5px 10px;
width:488px;
float:right
height: 314px;
font-family:Arial, Helvetica, sans-serif;
}