There's more to an overclocked graphics card than plain and simple GPU bin-sorting, and Asus' Radeon HD 6970 Direct CU II shows us exactly what that is: cooling.
After the graphics processor itself, the cooling design of a graphics card is the single biggest threat to overclocking performance. Your GPU may be able to crank out stupidly fast core speeds, but if your cooler can't displace the extra heat generated by an over-stressed chip, then it's all for nothing.
Even worse, your GPU may stop being friends with you and withdraw your pixel-pushing privileges for good.
You'll also need to keep your graphics processor fed with enough juice to help it go the extra mile, so a good power design is also vital if you want to get the most out of your card.
Asus has taken all this into account and produced the Direct CU II series of cards. There's already been an Nvidia GTX 580 version, and here we've got the AMD balance in the shape of the HD 6970 Direct CU II.
This series is all about the overclocking headroom.