Go for the 7970, specially if you can get the Gigabyte SOC version or the MSI lightning. I've a friend that always worked with ATI and he told me that most crossfire's have a lot of driver issues. Good luck
6950 is a great card.i know a lot of people will suggest 7970 but 6950 CF is also a great setup(only if you already have a 6950) otherwise [strike]7970[/strike] gtx 680 is a better choice.