Ok guys, I`ve now tried something around. With Yorkfield CPUs (Q9650/Q9550 etc) the option does not appear in the Bios when pressing ctrl+F1. With Kentfield CPUs (Q6700/Q6600) the option appears in the Bios, but it does NOT disable any cores. With socket 771 dual core server CPUs (like Xeon 5150) the Bios option appears and cores can also switched off!