I've had some luck with this program - http://coolbook.se/ . I mostly use it to lower my voltage, this rev a MBP just runs too hot/noisy without it!
I did disable the 1GHZ throttle speed and made 1.33GHZ my lowest, but I still notice my computer dropping to 1GHZ occasionally. Maybe the program isn't 100% up to par but it gets the job done
If you set the voltage too low you will crash your system - but after you find what your minimum voltage is you can just go up one or two notches and you won't have any problems.
The documentation should have some more information