If you want, Look into getting a GTX 570. for $310(you can spare 10, surely) after rebates, its the better choice of the two, unless you like to be dangerous and try to flash the card to a 6970.
Since you only use one monitor, that is my recommendation. No need to SLI if you are only using one monitor.
However...
Two GTX 460's in sli will give performance almost equal to a GTX 580. they run about $150 each, or $250 for a 2win version, which is one card that has two gpu's. While it will be louder, and run slightly warmer, you will save money.
Assuming your power supply isn't crap, the last option is cheaper, just uses more power. a 700 watt power supply minimum I would say, or a very good 650.
If you don't have the power supply to handle that, a GTX 570 for sure. In single monitor warfare, Nvidia wins the bout IMO.
*EDIT*:As for DX12, don't expect a card like that for another year or two. Even IF windows 8 supports it, they need to get the technology perfected in a way they can reliably mass produce them. For now, worry about what is out now. Maxwell(2014) may be the first DX12 card, Kepler and Southern Island definitely are DX11.