"Entä" includes a comparison to something that the person who the sentence is aimed for has been presented with earlier. Without any context, it doesn't really make any sense.
"Entä nyt" would be a kind of sentence you shout to your friend when you are on the roof trying to adjust the antenna to point in the right direction and the friend would be on the ground looking for when the TV gets the strongest signal. "How about now" would definitely suit that one better, and "now what" would be better translated into "mitä nyt".
It is true, like Juha said, there is certain minor cases where translating "now what" with "entä nyt" would work. As said, when asking where to turn in the intersection after recently asking the same person where to turn in the previous one would be one... And I cannot right now come up with a second one, so that kind of situation does not come too often.
That would be more like mitä.
Entä is more about the how. "This was this way before, how is it now?"
Mitä is more "I did this thing before, what I am going to do now?"
But I think they are more or less interchangeable.