Hello, people! Vader's voice is entirely different from Anakin's, so it is not a simple modulation. It is an entirely new voice created through a program. Therefore, anyone can have that mechanical voice if they want. Grievous will be the first we see to have it. General Grievous has the voice of James Earl Jones! It has already been taped and is circulating on the net. He says lines like. "My army has been defeated, but I will not go so quietly. I have killed dozens of Jedi simultaneously; what chance do you have?"
BTW..........................................
JUST KIDDING! Seriously though, if you believed me, then you deserved to be fooled. That was for taking credit for Ricky Gervais' work.
Okay, on topic. Though I am a big fan of John Dimaggio of Futurama, I don't think that he should be considered for Grievous' voice, contrary to many people who think he is in consideration.