is it wise to enable cpu speedstepping (EIST)?
 

on an overclocked PC. It has a stock heatsink.

Will I notice any difference?

it really depends on your CPU

im running an intel e8400
stock was at 3.0Ghz
i overclocked to 3.87Ghz however i have an aftermarket cooler
it runs WAY faster

depending on your CPU, you may be able to overclock a fair amount with just the stock heat sink (such as the Q6600)

i would suggest searching online for what other enthusiasts have been doing and results they have gotten.

I don't think clinton understands.

I think it's worth it to keep it on, once you find your max overclock setting. Turn it off while overclocking, then once you've settled on a speed, turn it back on.

This. Then I turned it off again for no real reason. To answer your question, no it doesn't make a difference.

I leave mine off. I can feel the response is slower at the lower CPU speeds used at light loads. It's about a 25% drop in CPU speed under light loads. If you are an overclocker I doubt if you would care about a saving of 20W on your Hydro bill.