x2 GTX 570 will give you better performance though you should get dual Radeon HD 7870 instead which are cheaper or around the same prices and perform better the GTX570, you can use this link to see the comparison between the two graphics cards- http://www.hwcompare.com/12208/geforce-gtx-570-vs-radeo...
The issue with the 570 is most of the time its 1.25 GB, and while that is barely enough at 1080 p (1GB is simply not enough, as shown by the 7850 1GB's terrible performance when settings are cranked up at 1080p).
1.25 GB simply won't last very long, and even in games such as Skyrim, even lightly modded your FPS will tank.
Not to mention by SLIing you'll more than likely run into SLI issues, such as scaling, and whether the game even supports it or not. Always go for a single card solution -- this also opens up the possibility of then SLIing the 670 again later.
The 670 also comes with 2 GB of video ram. Its also newer architecture (More power saving, while gaining performance).
That Hwcompare link won't show accurate results, as different architectures of cards will perform differently in different scenarios. Its like comparing clock speeds between AMD and Nvidia cards -- it simply is a stupid way to measure how good a card is. Although the 7870 is much better than the 570, no doubt. Since the 7850 almost defeats a 580, you can see how a highly factory clocked 7870 (MSI Hawk version comes to mind), would do. I would recommend a single 670, but if you must go for CFX/SLI, go for two 7870's.