You need higher voltage to support the higher frequencies.
Pushing the GPU past stock frequencies requires "extra voltage", and will also lead to slightly higher electricity costs.
Right now my three Gigabyte 6970s are set to 940Mhz with PowerTune at +5% (They have a higher voltage and 920Mhz clock by default), they get 420Mhash/sec.
My three Visiontek 6970's are reference model cards, thus with a default clock of 880Mhz. They too are running at 940Mhz, but since their default voltage is lower than the other cards, they need +15% PowerTune compared to my other cards at 5%.
The Gigabytes have three fans, but run much hotter because they do nothing to exhaust the hot air out of the case, so if you are reading this and are considering buying 6970 models with three fans, you might want to rethink that and get the single fan models.
My Visiontek cards run 25C cooler on average.
Gigabytes run from 80-90C
Visionteks are around 55-60C in a similar configuration.
So you will need to figure out whats right for your cards overclock in terms of voltage, because one cant properly overclock a card without setting the voltage with the newer cards.