RE: Palpatine dialogue - a new voice actor might not be necessary. In the last few cutscenes for the first Force Unleashed video game, Palpatine has some dialogue that could work, plus it's delivered by a pretty darn close voice actor:
http://www.youtube.com/watch?v=mRK6U9tZxyI
http://www.youtube.com/watch?v=MNIU-9X5Eso&feature=related