What happens if McCain dies?

This isnt a question designed to make people angry, just a quetion about the process. What happens if John McCain dies before the election? Does Sarah Palin become the Presidential nominee and would have to pick her own V.P. running mate? Would the G.O.P. have to step in and replace McCain with whoever they thought best? Would that leave her as the V.P. running mate, or would the new guy get to change from her to whoever they wanted? Please answer without partisan insults, or actually hoping that it happens just cause you hate republicans... Oh, one last question. The Democrats and Republicans have different sets of rules for electing their nominee, would the same rules as the question about McCain be the same if it were Obama that something happened to?