well honestly... some people stick with black/white mage BECAUSE of high job levels. I haven't gotten the third crystal on the DS version, but my white and black mage are both about level 60 in their class. Starting over with a level 1 job, you'll see obvious decreases in their power. Though if you get the new job to 60, you'd do just as well.

Oh, and if you want sage/summoner, change the white mage to the sage I'd say. Your sage will become more of the healing class, with a few black magic spells, while your summoner will be the damage dealer.

I already know my final party physical attackers will be a Knight and a Ninja. I'm not sure what my other two will be, but I'm leaning towards a Devout and a Summoner.

THE JACKEL