DDR2 800 has to be set in the BIOS on some 975x boards, ours being one of them. For the ASUS P5W DH Deluxe, you can't set the memory speed, you have to set the DRAM:FSB ratio, which I believe is 5:4 by default. You can increase it to whatever ratio you want in the "Jumper Free Configuration" category in the BIOS, but make sure the ram timing and voltage is on auto unless you want to mess with that yourself. Either way, you probably won't notice much of a difference since RAM speed faster than your FSB, in theory, shouldn't do you any good.