If you're keen on the green team, then I'd vote 670, as TG says, good oc headroom. But, I'd personally go for a 7950 'cos they're (with the state of current drivers), quicker than both in my experience.
A man called Shirty once said: "Hardness is irrelevant, it's just to seal a very small hole"
Keyboards, various. (More than I have hands and feet...)
i7-3770K // P8Z77-I Deluxe // 8Gb Ballistix // 770 // 256Gb M4 // 250D // AX850 // 3008WFP // 2i2 // Rokit 5 G2