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.
"If tin whistles are made of tin, what are fog horns made of?"
Keyboards, various. (More than I have hands and feet...)
i7-3770K // P8Z77-I Deluxe // 8Gb Ballistix // 680 SC // 256Gb M4 // 250D // AX850 // 3008WFP // 2i2 // Rokit 5 G2