It depends on how much memory you have or are planning to have in your PC. 32-bit OSs can't support more than 4GB of memory, though it is recommended to not use more than 3GB since it may have issues fully utilizing or even recognizing a 4th gig. While 64-bit OSs do have a theoretical RAM cap as well, it is high enough that I don't even think it is possible to reach it with current hardware technology. I'm not even sure what the 64-bit cap is, I think it's something ridiculous like 128GB.
Obviously your CPU plays a part in the decision as well, since if you don't have a 64-bit CPU then you can't use a 64-bit OS, but if your hardware is so old that you have a 32-bit processor then you aren't going to be able to run FFXIV anyway, so this is a non-issue.
I'd recommend using a 64-bit OS and running at least 4GB of RAM, but preferably 6GB or more. If you aren't planning on using more than 3GB of RAM then 32-bit vs 64-bit won't make a big difference.