Right I'm inclined to side with cooldudesubho on this one here. A video card is only as good as it is stock. The way it comes out of the package and when its inserted within your machine. Furthermore what I'd like to say is this.
The 7870 is more poised to go against the 660Ti, The 670 is more poised to go against the 7950 however overtakes it in most cases and is poised against the 7970. And the 680 is such a poor value money wise that its often times better just to leave it out of discussion.
Also given the new light of frame latency slapped across AMD's face it becomes a more daunting task of figuring out who is honestly better. And what I mean by that is that even though AMD may have stronger frames in some areas to the idea that there could be a stutter or a lag between said frames creates an issue of what is the more smooth and consistent experience.
Now I'm not saying AMD is bad and I would most certainly say that there current cards are a very excellent value considering with a 7950 you get crysis 3 and bioshock infinite. However, I feel a bit more safe with going with NVIDIA here mainly because it consumes less power and less heat, typically they take up considerably less space within your case, and nvidia seems to be stronger driver wise although AMD has made great strides over the last little while.
In the end though I'd like to say that any of those choices and the games you currently are interested in playing will yield an experience where you couldn't tell much of a difference with the exception of a rare circumstance where maybe the stuttering would could out due to frames out performing your screens refresh rate.