think it defaults to 512/512 background or something really low like that--so if you haven't raised the background setting, you are taking that low 3D resolution (background) and stretching everything to your higher overlay (foreground). Set background to be equal to your foreground (or higher). A 2:1 ratio makes for sharp details, but it may hurt your framerate, depending on your system specs.

If you want to try the oversampling for the FSAA, go /heal inside a telepoint crystal (crag of Dem/holla/Mea) while in first-person view and pan your camera around and such to see if you notice a lot of skipping/lagging frames. It will be most noticeable while you are looking down into the base of the platform while standing inside the crystal. If the animation isn't smooth, then your system may have a hard time with the higher background resolution in heavily populated areas (abyssea camps, besieged, Jueno, etc.).