I tried a handful of x5660 and x5670 cpu's. The x5660 were easy to get to 4.0 Ghz (20 x 200) but would not go further for me. The x5670 I finally landed on goes 4.2Ghz (21 x 200) without much problem. It is a bit of luck on what you get, but historically people always liked odd multipliers like 21 and 23 and claimed they are more stable. I have not personally found any stability difference running at a 20 or a 21 multiplier, I am able to do both.