The storylines are always the same, although they did ban some episodes (like the infamous Electric Soldier Porygon) and cut other bits out if they are deemed inappropriate.
For dialog, they'd most likely start with a straight translation but then they will change some things if it doesn't make perfect sense in English. Again they might need to cut some parts to make everything sync up.
As for episode names, I think they are pretty much made up from scratch to fit the episode. For example the Japanese translation of episode 1-25 is "Don't Get Angry, Primeape" but they changed it to "Primeape Goes Bananas" which is a bit funnier.
And the Japanese for the episode "Come What May" is apparently "VS Charem! Contest Battle!!"
