Price performance goes to the 7950, those dual fans sapphire models clock up to 1200mhz easy and will blow past the 670,680 even the 7970. Roughly 1.28-1.3v on the 7950 should get you up to those speeds, bigger memory bus and more vram.
The newest generation of GPUs from Nvidia were designed mainly for gaming , and any computing performance for anything other than games was slashed , so if you intend to use a lot of GPU computing in software "like cs6" you'll have to go with the HD 7950 .
. Less focus on compute. All of Nvidia's changes have resulted in what is, overall, the fastest and the most electricity-bill-friendly single-GPU gaming video card we've yet seen. But this title hasn't come without one sacrifice: compute. Fermi GPUs were sold, at least partially, on their ability to perform mathematical calculations à la CPUs, and displayed impressive facility doing just that, but Nvidia stripped some of those abilities away in order to improve power efficiency. Using LuxMark 2.0, an application designed for testing OpenCL compute performance, we compared last generation's GeForce GTX 580 (based on an updated Fermi-style GPU) with the GTX 680, and the earlier card came out ahead in every test—and AMD's new cards, like the Radeon HD 7970, did even better. If you want a card that's every bit as good for work as play, Kepler-based GPUs may not be the way to go.
You will also get a lot more value for your money with an HD7950 , getting a factory overclocked model with a custom cooler will cost you about 320$ , getting a factory overclocked GTX 670 with a custom cooler will set you back about 420$ .