6970 and CF it later.
With the latest drivers, the 580 is not better then the 6970 anymore. It depends more on the games you want to play. They trade blows in different games.
2 unlock 6950 in Crossfire would rape any 580 on the market, no matter how many VRAM it has, at that resolution.
Or else, buy the 6970 and buy yourself something nice with the money you will save, like a SSD (Crucial C300 64Gb for around 100$ for example).