HIS HD6770 IceQ X Turbo Crossfire Review
3D Mark Vantage
Considering that a single HD6770 is £100, so two are £200, and the HD6870X2 is £300, the performance seems very linear relative to the price. However if you look down to the bottom of the graph you can see that two HD6870s, current about £120 each, are quite a bit ahead.
3D Mark 11
The Crossfire scaling really shines in 3D Mark 11. Performance isn't fantastic but for such ageing architecture the IceQ does all right.