Hi, modern CPUs have idle and load clock speeds. If setup correctly I think you have the best of both worlds, for example my i7 2600k idles at 1.6GHz and is setup to over-clock up to 4.5GHz. Further more the Vcore is reduced at idle settings. If I set it to "AUTO" (stock speed) then it would climb (IIRC) between 3.8GHz with one loaded core and 3.5GHz with all cores loaded, this is Intels "Turbo Boost" in action, there are lots of CPU speeds/voltage that it does automatically switch between from 1.6GHz to the max the CPU allow for locked models or the max set for unlocked models. I think under Windows XP you need to set the power management mode to "laptop", Windows Vista/7 do it automatically. When over-clocking you can set fixed voltage this would give the same high load voltage even at idle so I use the "offset" mode, this increases the Vcore at all speeds, so idle does consume slightly more than at stock but it's not a bit issue, the CPU fans don't need to run at all at idle!
I think AMD had similar technology in their latest CPUs and I think Intel's Ivybridge can take this further than Sandybridge, ie more high clocks when temp / power consumption allow. If you're under-clocking an unlocked Intel CPU gains you nothing as the locked models are only locked at maximum, not minimum. If you have temp controlled fans I'm not sure why you'd want to under-clock a recent processor. Thinking about it I'm not sure the BIOS on my board allows setting a lower maximum, Windows power management does I think allow setting a max CPU state (in %) that may do it for you. The only reason I can see for this is if you have limited cooling capacity and want to limit the CPU power so it never ramps up. I would guess a low-end Sandy/Ivybridge CPU with a decent cooler will not need to ramp up. In fact I'll know soon as I've order parts to build a G840 based PC that's going to get the Freezer 13 cooler I have on it. I'm expecting idle 600rpm (92mm fan) to be enough even under load, and properly quiet too.
My rig idles really quietly, lots of fans but the idle very slowly. 2x140mm CPU fans @500rpm, 2x 92mm case fans @500 rpm, 2x 92 GPU fans, again 500~600rpm and 80 PSU fan also ~500rpm. Playing Batman Arkham City doesn't excite the fans much at all but Folding@home CPU & GPU does quite a lot. Not surprising as it's then nearly 300w DC beast
Does this help?
i7 2600k under NH-C14 w/2xTY-147PWM fans, P8P67Deluxe, 8GB RAM, GTX560Ti OC w/TwinTurbo II and BIOS fanspeed mod. MX100 512GB & 2TB EARX in Scythe QuietDrive outside case. Antec Signature 650 fan swapped. CoolerMaster 201C with 2x92 + lower 92mm fan out rear, holes-in-floor and mesh top+front intakes.