Try reducing the width of rightCont to 221px. Make sure you take into account the width of the borders and reduced the main content section by 2px (width)
HTML Code:
.rightCont {
width: 221px;
height: auto;
border-top: 1px solid #333333;
border-right: 1px solid #333333;
border-bottom: 1px solid #333333;
float: right;
}
.content {
width: 768px;
margin-right: auto;
margin-left: auto;
padding: 0px;
}