an update I am still on 2704 and its still stable no crashes etc.

Regarding voltages both C1E and speedstep will reduce the voltage on idle the difference is how they increase the clock speed on activity.

C1E - I believe raises clock speed/voltage whenever the processor is taken out of HLT.
speedstep - requires cpu utilisation to be above a certian % before it adds juice.

so C1E is less impact on performance but in turn saves less power and heat.

If you enable speedstep and C1E is disabled I dont know if it still reduces voltage alongside the clockspeed I cannot remember