Ah well, I thought you had 800 sticks. Too bad, some people had performance issues with 667 mem while overclocking in the past, so make sure you update to the latest bios (0302).
You might want to try fsb 400 as well, but raise Vdimm a bit (1,86V maybe).
And no, the "substract 10C rule" is for coretemp-based programs only, you can use coretemp as well since it's free