Shouldn't you just have the header in a container?
Code:
body {
margin: 0;
padding: 0;
text-align: center;
font-family: Trebuchet MS;
font-size: 12pt;
color: #556A87;
}
#container {
width: 500px;
background-color: #fff;
background-repeat:repeat-y;
margin: 0 auto;
padding: 0;
text-align: left;
}
#header {
margin: 10px;
padding: 10px;
background-color: #FFFFFF;
width: 500px;
border: 1px solid #999999;
float: left;
}
That should do it, I usually use that for layouts that need to be centered.