Don't go with Nvidia, AMD offers you more frames for your buck. I'd recommend one of the mid-end cards in the latest 5000 series, like the 5770. You'd be wasting your money on the outdated yet still relatively expensive 9800GTX.

Though with that kinda outdated CPU and memory I don't know if the GPU is gonna be the bottleneck here. Then again I don't know much (nor do I care) about FFXIV and how much resources it requires.