AT maximum overclocks they are roughly equal to each other, IMO. At least at 2560 resolution, they're about the same depending on the game. 7970 wins some 680 wins some, the difference is negligible most of the time if they are both oc'ed to their respective maximums.
680 will win bf3 and skyrim, bc2, sr3, dirt 3. 7970 wins witcher 2, all crysis games, metro 2033, alan wake, de: HR. 1300 might be doable on an accelero cooler, but you'd have to up volts and you'd dump a ton of heat in your case. 1200ish is a more realistic expectation if you're using aftermarket cooling....maybe more if you want to up the volts
The hardocp review has a good max OC comparison, also legitreviews / hardwarecanucks has a MSI lightning 7970 review, although the oc is a modest 1070.