If you overclock (or have a fast CPU) for performance, and you USE that performance, there won't be much idle time for a software CPU cooling program to help you in.
THAT is the fallacy of software CPU cooling - though billed as essential for overclockers, it does little on a stressed system.
On my old system though (Cyrix), I DID use a software cooler - RAIN - it improved the idle temp, not so much of the fanned CPU, but of the unfanned heatsink of the old-stlye linear mode voltage regulator, which ran uncomfortably hot!
A minute after activating RAIN, the heatsink was cold metal, as if it was doing nothing.
On my current system (Athlon on AMD750 chipset) none of my available software coolers made any difference - and the idle time in the standard Windows monitor shows the same effects as when using a HALT-based cooler - CPU reads close to 100%, as the idle time is eliminated.