Strange. Run a GTX460 and have no problems with it, but the problems you are describing are consistent with not enough power and/or over heating. I will say that FF14 will push a 460 to the extreme edges of its temperature range if you let it, so just to rule that out, here are the settings that I use (on an i5 / GTX460)

General Draw Quality: 8 - Extremely Important! Don't set it above this! (Can provide technical reason if you like)
Ambient Occlusion: Off
All other settings on/max (AA set to 2x for best performance)
nVidia Control Panel: Force Vertical Sync for FF14

Keep in mind, this gets me about 40 FPS, so its not optimal. This runs my GPU at about 68degrees. Lower AA to 2x and turn off depth of field for better performance. In the mean time, I would suggest HW Monitor or another program that can log your peak GPU temperature. Grab a can of compressed air and clean out your system. It is possible that there is a problem with the PSU itself, but these are easy things to test / fix before replacing the PSU.

Also, hit up nVidia.com and see about getting the newest drivers that came out about three days ago, see if that helps. Finally, make sure that you aren't running any other GPU intensive programs in the background. Photoshop, Windows Aero Overlays, etc., etc.