That is a silly way of looking at it. What if the gtx 670 you buy is a terrible overclocker, but the 680 that you might have bought instead was a great clocker. You then might have had spent more but got a card a good 30% faster. Obviously the opposite might happen and you could have saved a decent amount but it is all just a lottery.
Therefore it's only really worth comparing them stock for stock in my opinion(unless it's a factory overclock).
Therefore the 770 is a decent amount faster than a 7950. Even a 660ti or 760 can match a stock 7950.
I see your point, but the reference 7950 is underclocked - AMD wanted to show off low power usage (especially compared to things like the GTX 580) as launch. I doubt anyone would buy one that isn't factory overclocked, aside from the appalling reference cooler. NVIDIA tends to release cards with higher reference clocks and less OC headroom -- obviously this isn't a rule, but it's more typical than not with recent AMD cards.
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