[cherie]

Playing with the demo with Firebug here are the changes I made:

#page-container {
    width: auto;
}
#content-container {
    float: none;
    width: 960px;
    margin: 0 auto;
}

It's hard to tell about margins and such with the demo header in place but this should point you in the right direction. Firebug will help you track down which changes to make.