Dawn of Souls has upgraded graphics and a lot of new gameplay elements added to it. So yeah, if you own the original NES version, buying DoS would still be a damned good idea.
The FF7 spin-off is called "Crisis Core". It's the only game that Square has planned for the PSP.
If you wanted to add more Enix games to that mixture, you could have said that you play as God while racing against time to stop a giant Comet from crashing into the planet and freeing the souls of the world from monster-infested dungeons. That would cover Actraiser, Illusion of Gaia and Soul Blazer.
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