HD5850 and and 5870 IMHO, are overpriced right now due to high demand.
If you want something to last a while, the 5850 may well be the card you want, but I would hold off till after the first of the year, and see if the price drops some. I don't know that it will, but it seems that right now, right at Christmas, is not the best time to buy. Wait a few weeks. Check prices then.