Actually, while watching Jeremy Irons on "Inside the Actor's Studio" on BRAVO, I decided he was my image of Sirius -- he has the 'beautiful, but wrecked' quality, and would match up quite well against Rickman's Snape...

I still have no idea who could play Voldemort -- you need someone able to project presence and malevolence without turning absurd, and that's not as easy as it would seem.