Truth be told the 7970ghz edition can overclock just as well as the 770, but the catch is if you get a lottery card or not. Some cards overclock higher than others. It just depends what you are lucky enough to get your hands on. When it comes to performance they are not really that far apart from one another. If you are playing at 1080p then going beyond 2gb is not needed at the moment, but who knows what the future will require. I will say that if you are planning to go above 1080p then you might want to consider finding something with at least 3gb of vram for semi future proofing. I've played at 1440p and 1600p. It seems more and more games are hitting or coming close to that 2gb limit at higher than 1080p. I personally go for the better deals. The 770 are priced very nice, but I have to say the 7950/7970 are dropping in price.
Most 7950's run off the same pcb as the 7970's. I went and started looking at everything price wise. There are some very good deals on gtx 670's, but I am not going to include it since they are being phased out. Still you could find a good deal on it. The best deal in my opinion at this point in time is this 7950:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202003
It is can overclock to a 7970ghz edition for $285 after rebate and comes with 4 games. It is running sapphire's vapor x cooling. It can overclock to around 1200 core/ 1500 memory on average as long as you don't turn on the boost feature. The boost is voltage locked for some reason which is strange, but the normal setting is not. I did notice most were able to get around that oc without touching the voltage either way.
When the 770 is overclocked it would be faster, but you are paying more. Also, the games could even be sold if you didn't want them for more money on the AMD side. I am always haggling for the best price per performance. I'd say the biggest con of all time for AMD is drivers. Nvidia is usually spot on when it comes to driver updates as quickly as possible. I am still a little ticked at AMD for delaying the crossfire runt frame issue another month, however they do seem adamant that the fix is complete. The less you spend on something; the more money you have to spend on other things.
Conclusion: There is no real right or wrong answer. Look at the price, performance, and what you plan on gaming resolution wise. Are you planning to go past 1080p at some point? These are the kind of questions you need to ask yourself. Be sure to take into account how long you plan to keep the card as well. I'm the type that upgrades every other series of gpu releases which is why I try not to break the bank on every upgrade. I like to look at shiny things, but you don't see me staring at the sun everyday....well nevermind. lol
PS- I am running two 670's in sli. They were an awesome price when they released, but the 770 is now the same price I paid per card. I am by no means a fanboy of either amd or nvidia. I go where the deals are.