I'm a fan of ATI's cards, but for the price tag i'd say stick with the 660Ti, or get another and SLI them. You'd get more bang for your buck that way.
The 7970 is marginally faster, but not so much i'd ditch a perfectly good 320 dollar card (You could try to resell used, depending on what you think you could get) for a 400 dollar card.
If you've got the extra cash, yes it's an upgrade, but then again if you've got that cash and want to really future proof, get a 690, 3rd party 7990 or wait for the official offering of 7990.