This is my current setting for the Xeon X3440, showing a stable overclock of 2.7 GHz. I tried to push BCLK past 142, but at 145 the system instead slowed down (using Cinebench, Geekbench and XBench), and at 150 I had a kernel panic and did not go past the gray Apple splash screen. I know that if I increase the VCore and Dynamic VCore, I could probably go past 150 and things would remain stable, but at this point I do not think I need it. I am happy to push my system from 2.53 GHz to 2.7 GHz without too many tweaks.
By the way, I did purchase an aftermarket processor fan - a Coolermaster 101i. It is one of the less expensive ones but as I have read good reviews about it comparing it to more costly models, I decided that $14+ tax is worth the investment. Even with the overclocked CPU settings and while running Prime95 for about 3 hours, this little cooler was able to keep the temperatures of all cores at around 51 degrees Celsius. Not bad at all.