seems everyone has covered the AC aspects. I will tell you I had no problems with staying in 270 upgraded shire gear on all 3 jobs I've taken through to 70 up till about level 66/67. 2DPS 1 tank. Dungeons were no problem, though I didn't try doing large pulls with my Tank... AND I had a chocobo companion out for almost all overworld fights.
I'd recommend grabbing the 66+ gear for your first job to 70, but keep in mind that once you hit 70 you're going to get a free set of 290 for completing your main job quest.