Becasue the q9550 has the FSB1333MHz, you use the 1333MHz RAM because the MB only support up to 1333MHz (OC), you need download the manual from company website. Go to the page 20 to see how to set the jumper so that you can use the 1333mhz RAM. http://www.asrock.com/mb/overview.asp?cat=Manual&Model=G41M-VS3