The difference is mainly in the memory bandwidth. A larger bus width (i.e.: 256-bit) allows for more data to be transferred at the same time. What's the link to the deal on the $200 card?
My general advice would be not to invest in a card that scores under 2,000 in this list
, particularly because of how demanding FFXIV is. (Again, the point being that if you try to go cheap now, you're only looking at making another upgrade soon down the road.) Those top tier cards will have probably 4-5 years of shelf life in them before you're to the point where you'd want to upgrade again, but most of those also get expensive quickly. Each dollar above ~$200 just doesn't bring in nearly as much performance.
Not like you're looking to spend that much anyway, but it's worth mention. Edited, Jul 30th 2011 10:10am by bsphil