I would probably say go with a 570 for your budget but if you do end up getting another monitor, you may need more vram. How long would you like this card to last? You may want to look at a 6970 because of its vram. Playing at those high resolutions will weigh in on your vram, especially in newer games to come.
If you're planning on definitely getting a second monitor and you need a new card now, get a 6970 and crossfire it down the line when they come down in price. If you're planning on sticking with the one, get a 570.
Or if you can wait awhile, the Keplers are likely to drive down 79xx prices and, if you're willing to move your budget up a bit, you could get a 7950 at $375-425 price point. Well worth the wait if you ask me.