highest is equal to what your machine is advertized to do (ie you get the 1.33GHz ibook, highest will be 1.33GHz)
as far as an actual number goes for reduced, that i don't know but would venture to guess for like the 1.33GHz ibook above it is like 800-1000MHz
I think the clock speed drops about 1/3. The itunes visuals drop about 1/3 in framerates on reduced mode. I also noticed that some games totally ignore your power settings and sun as fast as the computer will go. I dont know how they do this, but halo does it on my PB.