I purchased a set of TWIN2X-PC 8500C5. It boasted to run at 1066 MHz.
I could not get past 850Mhz. I talked to Corsair and they said it was not on the QVC list for the P5b. I returned it any for another pair. Same thing!
I sold it on e-Bay and purchase some cheaper 800 Mhz memory on the QVC list. What I have now will run faster at 2 sick, but now I am using 4GBs. I also had change my timings from 4-4-4-12 to 4-5-5-12. I don't mess with the other memory settings because I don't know what they do. I also changes my memory Voltage from a default of 1.9V to 2.3V. I read this on a review of my memory before I received it.