I will say HD 6870 slighly better than HD 5850.... slower than HD 5870 its not a successor but CHEAPER + power efficient
HD 6850 replaces the HD 5830 which really really crippled card.. Slower than HD 5850 (but not much)

Reason why ATI/AMD (whichever name you prefer) decided to name the 6800 series as they are right now is because the HD 5770/5750 is selling very well. AMD doesn't wanna hurt the sales by naming the product HD 6700 series. But I still think it should be named 6700. Anyway... I wonder what Cayman X 2 will be called....
Sice Cayman will be = 6900 will the X2 be called 6900X2 like the 4870X2?