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.
"The Quiet Rig": Maximus IV Extreme Z - i7 2600K - Frio - 16Gb Tactical Tracer - Sapphire 7970 CF
480Gb M500 - LG Blu-Ray - AX1200 - Define R4 - 3007wfp-hc - KRK Rokit 5 G2s
"The Other Stuff": 15" i5 MBP - hFX Classic HTPC - 2x Technics SL-1210 M5G - Sonos - Some Servers