#main_content
{
	z-index: 0;
	position:absolute;
	top: 50%;
	left: 50%;
	width:800px;
	height:590px;
	margin-top: -310px; /*set to a negative number 1/2 of your height*/
	margin-left: -400px; /*set to a negative number 1/2 of your width*/
}