Guys! Don't waste your time for such a discussion. When a borrowed word is considered, please keep in mind that it has already been borrowed and is in fact a word of Russian language. It is pronounced in some way by Russians, and it's spelled accordingly. The name "Jack" (a foreign name, in fact) is pronounced by Russians somewhat like "Djeck" and is spelled "Джек/Джэк". The word jazz is pronounced "djuz" and spelled "джаз". Isaac is a harder case; we have some jews here in Russia and the name is spelled and pronounced as Исаак. A hundred years ago and earlier people used to transform foreign names into their language (e.g. find the closest equivalent) - that's why Isaac Newton is Исаак. And recently a new tradition has developed - foreign names are just spelled by Russian sounds. That's why Isaac Azimov is Айзек.