"Теперь" is used when something has changed or is going to change. As it-ogo has said it is often used when we tell about consequences of some events. It is even possible that it doesn't mean "at present" or "nowadays". For example "ты получил двойку и теперь не поедешь в Диснейленд". It doesn't mean that they were to go there immediately. The travel could be planned for the next month. It is implied that the plans has changed now.

"Сейчас" on the other hand means just "at present". "Сейчас мы не пойдём гулять, сначала выучи уроки". You shouldn't use "теперь" in such contexts. It can sound old-fashioned.