I've used both nVidia & ATI/AMD -- my primary issue with ATI/AMD is the GPU's lifespan, I've had 5 AMD failures to 1 nVidia failure. Next, I prefer and use 3D Vision and I've setup and used HD3D the 3D Vision offers better game selection and a better + consistent 3D experience. The reason is simple, nVidia is very controlling of the hardware and certification whereas AMD is not.
Now 'IF' AMD can levy higher control over it's manufacturer's (quality control) and peripherals (quality control) then maybe the reality of it's problems will be on par with nVidia then and only then will it be about FPS.
Clearly, OC either the GTX 670 or GTX 680 -- non-reference cards and run (rerun) the comparisons.
Chris - You already tested non-reference GTX 600 series cards, Why
DIDN'T you include the data?? Makes me think 'agenda' or blatant oversight. If I'm comparing OC or SOC then I'm going to compare Apples to Apples and not Apples to Bananas. I like your reviews when they're on the up & up...update your charts...you have the data.
Example reference vs non-reference cards:
ref -
http://www.tomshardware.com/reviews/geforce-gtx-670-review,3200-2.html