I would get a 570 gtx or a 480 GTX super clocked which has slightly higher performance than a single 570 GTX. Or you could get a ATI 6970 for the same price. Although I'm a huge Nvidia guy so I have a slight bias towards Nvidia. But I can tell you this that I don't like AMD drivers because they are so glitchy with textures in some games(AKA some updates make the textures look like rainbows and some fix it). I have never had an Issue with Nvidia Drivers.
Depending on where you look either at Tigerdirect or Newegg, they both have all those cards for about ~330-350$
I would either get the HD 6850 or HD 6870 for now or a regular GTX 560 for now BECAUSE AMD is releasing their 7xxx series by the end of this year and nVIDIA will introduce their newer generation Keplar in 2012. In other words, don't spend so much now and wait for the newer generation that's coming out PRETTY SOON :]. If you're rich, you can get the 560 ti or gtx 570. They already established themselves as good performance although a bit pricey.