a 580 stock is about 9-10% faster than a 6970 stock @ 2560x1600 (from looking at TechPowerUp).
The price differnce for those 9-10%?
6970 ~315$ on newegg.
580 ~480$ on newegg. (about 9-10% faster at 2560x res, cost ~53% more)
If you want even more value, you could buy a stock 6950 and flash it's bios.
The 580 is the fastest single gpu card, but you pay through your nose for that little differnce.
So Id recammend a 6970 or 6950.