it won't matter if you play any of the other ones because each numbered Final Fantasy revolves around a different universe with a different set of characters. Final Fantasy XIII is a new adventure so you won't be missing any story.
It believe its difficult for a lot of people to go back and play the older PS1 titles (due to now outdated graphics) but I would recommend FF7 (my favorite), then FF9, and FF8 in that order on PS1. All PS1 games are backwards compatible on every PS3.
For the PS2 versions I really enjoyed the turn based FF10. 10-2 wasn't so good story-wise but the active battle system was fun. You could stop enemies from attacking you with a quick attack while they were running at you. Finally FF11 was online only game with a subscription based model so I never played it. And FF12 is all in real time, you see every enemy wandering around. All the other games would just send you into a battle screen while running around. So in FF12 you can easily choose whether or not you want to engage in a battle. A lot of people like this one a lot, but for me the story wasn't as engaging as the previous ones.
I hope this helps. We still have a long wait for FF XIII & FFXIII Versus. I really hope FF13 makes it out before 2010.
