Can you easily notice the difference in game between a 580gtx/6970 and any of a 7950/7970/670/680, sure, without question.
Will most people notice much difference between a 7950/7970/670/680 assuming the "lower" cards hit a similar overclock, no. There will be a few games where Nvidia are way ahead, and likewise some where AMD are way ahead, but on average you're basically talking the same ballpark performance for these cards, switching from one to another is madness.
Frankly the only reason to get a 7970 or 680gtx over the "lower" end card is if you get a stupidly good deal, it will be worth more when reselling and it will be a bit faster, not that you'd really notice it.
The ONLY time I'd switch, ever is if I got heavily into one particular game that I'd play literally all year and there was a noticeable difference between Nvidia/AMD in that particular game. Max Payne 3 might be 25% faster on Nvidia, but you'd probably not play it for more than a week, and the next game out is give or take same performance on the 680 and 7970.
Even then I personally wouldn't bother unless it also made financial sense.
I'd have a 7950 if the 7970 price I got wasn't so awesome(as in £15 more than a 7950).