Is it a 5870? (Link doesn't work)
What are you coming from?
Are you willing to OC your GPU, its literally as simple as moving a couple of sliders and completely reversable? If you are the 5850 or the 1GB GTX 460 may be a better option. Once those two GPUs are overclocked you won't actually lose much performance compared to a 5870, but you'll certainly save a nice stack of cash.
Your PSU is fine for any of them and honestly, all of them should run FF14 like butter, the game's capped at 30fps afterall.
The GTX 460 looks like it might be coming in right at the price

erformance sweet spot, more than enough power for high end gaming and yet no need to pay an extra premium for it. It very well may be my defacto high end GPU recommendation as the 5850 was once.