Keep in mind that over-cloking the processor will void the processor warranty as well as using a third party cooling solution, Intel recommend using the stock cooling solution.
If you still want to do over-cloking, it will be under your own risk, regarding the processor temperature, the maximum recommended temperature for your processor is 67.9 degrees Celsius, so as long as the processor runs below this temperature and under designed clock speed, it will be running under specifications.
You can double check the maximum temperature at:
There a several reasons why the idle temperatures with your Noctua CPU cooler are not lower than they are, although they are not bad now.
Do you have EIST (Speed Step), C1E, and other C-State CPU power saving options enabled in the BIOS? That will lower it's clock speed to 1.6Ghz at idle. If your CPU is running at it's standard clock speed, 2.8GHz, when the PC is idle, you'll likely have temperatures like you do now. When people quote very low idle CPU temperatures, they are almost always using the CPU power saving options. I use them, they work fine and immediately let the CPU come up to it's full speed when needed. At idle my CPU temperature can drop below 30C at times when the power saving options are on.
What type of fan speed control do you use? I believe the fans on your cooler have three pins for power and control, so you can't use the automatic four pin PWM fan control. Your mother board likely has three pin fan control software, which can be almost as good as PWM. If your board has fan speed profiles for the three pin fan connections, you should try a more aggressive profile that speeds the fan up more at lower temperatures. Are you using the low noise adaptors with your fans? If so, the fans may be running to slowly to cool your CPU to the extent you desire. You should try removing them to enhance cooling.
A great CPU cooler like you have will work at it's best only if the air within the PC's case is as cool as possible. How many case fans do you use, and what size are they? If hot components like video cards, multiple disk drives, etc, cause the temperature inside the case to be about 35 -40C, any air cooled CPU cooler will not be able to cool a CPU below those temperatures.
Over clocking any i7-900 series CPU is not simple, and requires multiple adjustments and tuning of BIOS settings in order to work correctly. Your problem sounds like a memory problem caused by raising the BCLK frequency to change the CPU frequency, but that also raises the frequency the memory operates at, and the memory multiplier must be reduced to compensate for this.