For overclocking today we used the excellent Sapphire TriXX software tool, which we use for all our AMD based discrete reviews.
Above, the HD6790 DD, HD6870 DD and HD6950 DD.
There is clearly some decent overclocking headroom with all three cards, averaging around a 8-10% increase depending on the model.
If you make the effort, there are considerable performance gains available with a little manual ‘tweaking'. Temperatures weren't affected on any of the cards as we didn't need to increase the voltage settings at all.