With the recent driver update, no need to get the GHZ Ed since the normal 7970 has major improvement and OCs very well. I'm tempted to sell my GTX 670 and grab a 7970, 2 of them on black friday!
Unless you are unsatisfied with your 670 in any other way than it being a touch slower than a 7970, I would not advice switching. I've had a lot of cards over the years from both companies, and at least half of the cards I've owned had little annoyances, such as noise, heat, bugs or what ever. It would be a shame if you sold a card that worked well only to find an annoyance in the new card.
Now if you had little problems with your 670, then go for it.