I have an i5 750 running at 3.16GHz, and the voltage is 1.168V.
During CPU idle state, the CPU runs at 1.42GHz, the voltage is constant at 1.168V.
During full load, the voltage rises to 1.184V.
Does CPU need 1.168V during idle? How do I lower down the voltage during idle state?
You need to have C1E enabled to lower the idle voltage, and you can't have Load Line Calibration (may be Vdroop control, depending on how your motherboard labels it) enabled, either. It sounds like you have LLC enabled, as that's the only reason it would raise the voltage above what you set it at. I recommend disabling it if you can, after reading an article from Anandtech yesterday: http://www.anandtech.com/show/2404/5
So now I have LLC disabled, turbo boost disabled, C1E enabled, C3/C6/C7 enabled, and EIST enabled. Is that my settings?
BTW, what is Bi-directional Prochot? (I want to learn more about these)
And what is the best voltages that I have to set?
Well, voltages will all depend on your specific chip. If I had a 750, it would most likely need a different voltage at the same settings you have.
The reason it never reaches the set 1.168v is because of Vdroop, it's normal.
However, if during full load it does not crash with 1.136v, then you're good to go! I'd even start playing around with other voltages and see how low you can get them all. Lower voltage = less heat and more stability.
And as for the bi-directional thing, I looked it up, and it looks like it is part of the thermal throttling communication line, ie, the CPU gets too hot and reports the temperature to the motherboard, and the motherboard either throttles it down, or shuts it off if it is TOO hot.