First thing, would be to set your General Draw Quality to 8. Anything over this setting is just going to tax your GPU for no reason. Once this is done, play with your other settings. By lowering my General Draw Quality from 10 to 8, i was able to turn on all shadows, max AA, and crank up all other settings to max, and STILL had a performance increase. The short of the long, is that there is no graphical improvement in having that setting above 8, just a performance hit.

After you try that, then consider upgrading. As it is, I run an i5 quad core 2.8gz, and GTX460 (150$ card), with all settings except General Draw quality maxed. That being the case, there may be other reasons your computer is going slow (harddrives adversely affect FF14 as well, for example). If you post your info, we might be able to help you more.