Hi,
It depends on your budget really. I would suggest something like a 6600 as they are reasonable on price but would be fine for the game you are trying to play.
If you can not go that high on money, then you could try a 5 series card instead, again it should be fine for the game, just not quite as good as the 6 series cards.
What is the full spec of your machine, as it depends what the other parts of your machine are as to the graphics card you need really.
Rick.