That's good news, and I hope he has a VERY powerful role. He desserves it... Cushing deserves it as a sort of homage to him. He was such a superb actor. Second only (And not even in my personal opinion, but that of the public I think) to Sir Alec Guiness.
Tarkin's character needs to really be someone of power and intellect though. Not a yutz like some of the villains, but someone who's equally as vicious and evil as the Sith Lords, and maybe even knows what's going on somewhat.
He certainly seemed to know much about Vader's past in ANH, and was ok with how things turned out obviously, so I cannot wait, and I have big hopes for this guy and his eventual figure.
Nicklab's idea for the actor is good too... British, evil looking, definitely a resemblance to Cushing, and is already known to be a good actor. Can't ask for much more.
The "Buffy" guy Spike doesn't strike me as quite as nice a choice.