there is a final fantasy pen and paper RPG on the net. It isn't d20 though.
As for D&D, I feel 4e translates a lot better for FF than 3.x. (check my blog for more info).
My suggestion is to use the FF PP RPG as a guide. Of course the scale is much different. Basically, I would divide all of the levels (when you can get powers and such) by a factor of 5 (corresponding to level 20 = level 100 in FF) OR a factor of 4 corresponding to level 64 (the last level to gain new abilities) to level 16 in d20. As for races, I have thought about it before, and came up with some numbers. But again, I'd use PP rules as a guide. They are well done.
The MAJOR drawback to it is the monsters. There is no good database for monsters in that, but with d20, it isn't a problem.