However...further in REvenge of the Sith when Palpatine is talking to Anakin..he asks him the same question.."Anakin..was Dooku a Sith Lord or just someone out for power holding a red lightsaber."
I think from Dooku's realization and Sidious saying this that Dooku was not a Sith Lord.
would you consider Asajj Ventress a Sith Lord, because if you do, then that would make 3, not 2. Which would make the Sith rule not true. even if they don't follow rules, the still learned from their past mistakes. I think Dooku was mislead just like Ventress. Maul, from the history we have, was never a jedi and solo trained by sidious. Anakin, however, makes the ultimate choice when he slaughters Windu and the Jedi temple.