If you have a good quality PSU, voltage is what pretty much held me back, but once I got an OCZ, I was able to push this 650 all the way upto 4.42ghz on air but certainly not stable. Also RAM voltage, I never went above 2.1 but gave the full 2.3v.
Although I was messing with a lot of the settings in the BIOS I think the EIST feature and C1E were disabled when I hit 4.42 on air. But in the end voltage was a factor, I didn't fry anything though.
All Eist does is lock our multiplyer. It's main function is to control the speed of your cpu. If your surfing the net, not using much CPU power, it will run at ex 2.0, when your playing a hardcore game like Doom 3 or CSS, it will use 2.5 or 3.0 (saying your OC'ed to 3.0)
Off for OC'ing, On for safety and power consumption lessened
Personally for both default settings and overclocking I would keep EIST enabled.
EIST only affects the computer when it is in a idle state or if it is using very little processing power. Once the CPU is under heavy use EIST will not operate.
What Enhanced Intel SpeedStep Technology does in the Intel 6xx series processors is drop the standard multiplier down to a x14 multiplier. This reduces the clock speed but also allows the system to automatically reduce the system voltage. This reduces the power usage of the processor when its idle and obviously you have a reduction in operating temperature. When you start to use the processor again it will increase the voltage and raise the multiplier back up to its original value. The Frontside bus (FSB) remains constant throughout and does not change, so you will loose no bandwidth.
If you want to turn it off then do so, but I would personally leave it on as it will help when the computer is doing very little.
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