Hi the CPU compatibility list for that motherboard: http://www.intel.com/support/motherboards/desktop/d955xbk/sb/cs-026574.htm
The only reason why you can't and why it won't work is because the microcode of the CPU have not been inserted in the BIOS. myu guess it is simple marketing. SPecially with the likes of this board that are very very good. If tehy did update teh BIOS with all thge C2C and C2Q, you wouldn't have to buy another motherboard!