Before I go on a long song and dance about how to fix what I think you've got, first thing is whether or not you've gotten the terms refresh rate and fps (frames per second) confused. As the previous posts have mentioned, the game is capped at 30 fps, hard-coded in if I remember right. While "refresh rate" on the other hand deals with the actual timing of your monitor, if you've a CRT that is (LCD screens have a different term, called response times, but that's irreverent to this post). You might be familiar with the "flickering" of CRT tubes. That would be the refresh rate. (aaand, if you feel like it,
wiki on refresh rate.) If you're playing FFXI at a different screen resolution than your desktop, there's a chance Windows has defaulted to 60Hz, which, to me, gives me a blinding headache/migraine in a matter of minutes. There are ways to fix that, but it's late, and I'd rather not explain how if it's not needed (what can I say, I'm tired, and lazy).
So, do you mean "refresh rate", or did you confuse it with fps for FFXI?