Overclocking E6550 on P5KC board with DDR667. Impossible?

Hello,
I've been trying to overclock my old Core2 E6550 on an ASUS P5KC but my problem is this: Using DDR667 there doesn't seem to be any way on this board to raise the FSB above 333 default without overclocking the RAM too. Everyone else who has DDR800 is able to underclock the RAM to 667 and then up the FSB to get the 1:1 ratio but there's no option to underclock below 667 so my RAM seems screwed. I have the latest BIOS version and have tried lowering the FSB strap (200,266) but it just raises the available RAM frequencies, plus it prevents POST at any setting other than auto. Am I missing something or have I just got the wrong board/CPU/RAM combo for overclocking (seems many people have a 266base FSB, is this just certain CPUs?)

Cheers