You would basically have to rip apart your console and replace its internal components, then rewire everything!
Not really. Remember that the Cell processor is designed to be networkable and expandable. You really only need to hook up another one and it's twice as powerful. I guess theoretically there's no reason it has to be the same amount of power either.
I have no clue as to how to get the DLC!!
My PS3 is not (yet) networked, but I guess you open the PSN store. If it's available, it should presumably be there.