Have you read the RotS novel? As far as I know, that's the most we've seen of Dooku's motivations revealed. Dooku muses before his duel vs. Obi-Wan and Anakin that he will rule alongside Vader and Sidious in a stable, efficient and ordered Empire, well that's the plan anyway. I think he truly hates the bureaucracy and the corruption of the Senate, and that caused him to fall to the Dark Side. It's also memorable in that it has a scene where Sidious and Tyranus talk before the betrayal. Beauty.