The gtx 580, if you can afford it, is the absolute best option right now. It is better than a single 6970, and you can also SLI it in the future when you are ready to upgrade.
If you don't have $500 sitting around, go for the GTX 570. It is generally better than the 6970 and you can find it a little bit cheaper if you look.
The only reason you would want to SLi a 580 is for resolutions higher than 1080p in which case the 1.5GB of VRAM can definitely be a limiting factor.
The HD 6970 is a better choice for multiple cards and multiple monitors since it's cheaper and has 33% more VRAM.