Im getting my new PC today! And Im keeping my old graphcard XFX 6950 (2GB) but getting another one so going crossfire. However, 6950 is the same hardware as 6970 so you could actually make a 6950 convert to a 6970. (so if you want you can save some money).
Though, the new series is released around this years december (the new XFX 7k series, would be worth waiting for if you want the best of the best).
Geforce i'd buy 580, nothing less than that, for the best performance it will run around 90FPS in bad Company2 (witch is a good game to compare graphcards in). You could buy the 570's that is clocked, but I've heard that there are some problems with them so it's problebly best bet to go with pure 580 for great performance.

(think my friend saw 580 on amazon.com for around 3k SEK witch is like 3000/6,72= 446 USD
You can compare on this site:
http://www.hwcompare.com/9155/geforce-g ... -6950-2gb/