It's obvious isn't it? Regulus Black! He was mentioned twice. Once when Dumbledore was talking about who 12 Grimmauld place should go to and once in reference to Karkaroff surviving a few weeks after abandoning Voldy. Supposedly Regulus only survived a few days - long enough to take it.

I don't think that Dumbledore would have been able to kill Voldy due to the prophecy - although i had been pretty certain that Dumbledore was going to die during the final battle by helping Harry in the big fight.

I reckon Dumbledore died because he had fulfilled his purpose. Harry is now mature enough to continue on his own. Dumbledore has taught him all he needs to know. I think that Dumbledore's 'lessons' were actually his way of seeing if Harry was able to work out things without jumping to conclusions. (As he has always had an issue with that in the past.)