Take it with a grain of salt, but according to the rumor mill NVIDIA's GP104-based lineup may feature both GDDR5 and GDDR5X memory. As the rumor goes, memory bandwidth may be one of the key differences between the GeForce GTX 1080 and GTX 1070. Both cards are reportedly based on the GP104 die, with the GTX 1080 using the new GDDR5X and the GTX 1070 sticking to the old GDDR5.
This should give potentially up to 100% more bandwidth for the GTX 1080 and better energy efficiency to boot. Due to the use of different memory, the 1080 will boast 20 more pins. The 1080 will use the GP104-400-A1 and the 1070 the GP104-200-A1.
For two similar cards based of off the same GP104, there seems little reason to split the memory between two different types since the required memory bandwidth should be similar. One possibility is that the GTX 1080 may be using relatively slower GDDR5X that isn’t much faster than the fastest GDDR5.
This makes sense if GDDR5X is supply constrained for the top end models. Another possibility is that both were meant to get GDDR5X but supply meant only one of them could. Finally, it could also be a way to differentiate the GTX 1080 as it may remain the GeForce flagship for quite a while.
The report also indicates both cards will feature three DisplayPort, one HDMI and one DVI connectors. It also points to two 8-pin PCIe power connectors, which seems a bit much.