Both great cards , was reading the reviews on the 580, they say this card is a monster!
It all depends on you really , I do love the performance of the new 7950 but hearing about a few driver issues with the card , but they are very new and i'm sure the drivers will get better for them.
Would you be prepared to wait and see what the new 680 cards going to be like, might be worth the wait and to make your decision then as it would drop the prices on the current cards.
I've owned ATI for years, and currently switched over to Nvidia and have vowed to stay here. There have been too many separate cases of bad driver support for particular games (Borderlands, Monday Night Combat are 2 examples that come to mind) I have gotten hooked on. Overall, I feel that even though nvidia is more expensive, you're getting a better product with better drivers. I would buy the 580, that extra 1.5gb ram won't be needed by the time another upgrade is necessary imo.
I would but I already sold my current gtx 570 and I have to replace it with something soon. I don't plan on overclocking or doing sli or cross.. I don't know which card is really better overall in gaming.