Honestly I would just get whatever one is cheapest. Even if the OP is going to run at 1920x1080, the card is going to run out of juice before it can really utilize the memory of the card. If you are going to keep it for a short while that is.
Although, I would really take into account what the other two have mentioned if you are going to keep it for a while. Little things are going to make a bigger difference down the road.