If you manage for find any of these in stock for around $600: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
or any other brand 5970 for that matter
At about half the price, a 5850 is a good buy aswell. You could get 2 and crossfire them but you wouldn't equal the performance of the 5970
a single 5870 would get you more than a single 5850 but is $100 more.