Actually Birth by sleep takes place before the first Kingdom Hearts.
Though yeah, I don't think it hurts that much to play the games out of order. (Story wise)
Yes it would keep the story going in a straight line, though with all these games on all these different systems, it's kinda hard.
Kingdom Hearts 358/2 Days, was actually my first Kingdom Hearts game,
and I had no idea what was happening out side of that game's own story. (Which was pretty amazing, by the way.
Yet it was what got me hooked onto the series.
Oh, by the way.
I heard that Kingdom Hearts 3 will come out after one other Kingdom Hearts game.
(So it might be out within the next year.