Out of curiosity, when was the last time you replaced your RAM?
EVERY single time that I've ever experienced graphical glitches like that, replacing the RAM has fixed the problem.
Given that you are not playing on a potato powered toaster, and probably play higher end games than FFXI, I would imagine that if the RAM hasn't been swapped out in a while, it's probably been cooked more than it should be more than once. ^^;;