7970 vs 670

Fact of the matter is they're both good in their own ways. In overall performance its VERY close and the difference is not too noticeable. But you have to see what market you are aiming for. If its gaming then i would say Nvidia simply because at the current moment they're more supported by the Game Industry and i know this for a fact because i'm a game developer and I've spent countless hours doing research at what GPU's most programs are optimized including Game Engines. The architecture of the GPU is just much more streamlined.

On the other hand Radeon is far superior when it comes to video. By miles the Radeon will destroy Nvidia when it comes to editing, playback, and even audio if you wish to use HDMI. Some people even say that the HDMI audio that you get from Radeon's are just about the same as the quality off of a studio sound card. But also when it comes to shaders in games, you do sometimes see better performance in gradiance over Nvidia.

Overall, they're both good and they all have their pluses and minuses. if you're running Intel i would strongly suggest Nvidia because Intel MB tend to have better performance while an AMD board you generally see better output with a Radeon. I know i probably will get a lot of arguments against it but this doesn't apply to EVERY board. Simply some boards will bottleneck when they come across some specific GPU architectures that may have strings that possibly may clash with CPU strings.

So long story short, make sure the MB and the CPU work well with that specific Video Card. I know the performance at the end isn't going to be drastically noticeable but when it comes to reaching the limits of their performance (e.g. Overclocking, SLI/Crossfire, or even chaining), you may see conflicts within the CPU, MB, and GPU arise. P.S. I have heard that Radeon's sometimes have problems going Crossfire like the performance increase is so minor that you get as low as 5% boost in performance.