Why not use the Mono PLUS? I'm using it on my HD 6950, reaching 64°C maximum in Furmark. And it's very quiet, thanks to the 120mm fan (the twin turbo uses 92mm, the Xtreme III is to much for 670 imo).
The overlapping heatsink contributes to the airflow in your case, which is not so with the twin turbo afaik.