FFXIII will be the first one to come out, naturally. FFVXIII will hopefully come out in 2010. Also, from what I have read over at GameFAQs, S-E has made FFVXIII the core game of the entire completion (Fabula whatever.... xD). FFXIII was originally the core game of the completion, but I guess they made FFVXIII the core game to appease Sony and its fans...
Nevertheless, I'm not sure if this is true, but it is a rumor I heard with a possible link to show the proof. I'm sorry to say that I do not have that link or know the exact place I heard it.