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it depends really, the PS2 version has readable text and better framerate but a not so good CGI opening and voice acting. The PSP version has a neat anime intro but the font is small and the framerate is pretty laggy. I'd go with the PS2 version in that sense. Oath in Felghana is already released, they got a normal edition and a premium edition which comes with a soundtrack of the game.
which version of Ark of Napishtim were you after? psp or ps2, be careful when playing it because the framerate is choppy and the load times are pretty poor. As for the remake, the whole story was rewritten, the same things happen from Ys III in Oath in Felghana its just told differently and theres more things in Oath in Felghana that weren't present in Wanderers from Ys like extra bosses and magic attacks.
The new Ys games aren't like Ys III. There is a Ys title that just came out which is a remake of Ys III called The Oath in Felghana. As for knowledge of previous story, you don't HAVE to know what happened in the other games before playing the latest ones. It would be helpful though. What Ys game were you thinking about trying out?