A new image of the NVIDIA GP104-400 seems to confirm rumors about the GeForce GTX 1080 video card using GDDR5X memory. The memory chips on this card are marked as Micron 6GA77 79TXT but official specifications of these modules aren't available yet so we don't know the clock frequency. The GP104 GPU itself measures 37.5mm x 37.5mm, resulting in a die size of 317mm².
That said GeForce GTX 1080 might receive a substantial boost in memory bandwidth thanks to this cutting edge technology. Leaked picture confirms GP104-400 is using with Micron 6GA77 79TXT modules. Unfortunately those modules are not yet listed in official product catalog, so we are unsure of the exact speed of those chips. However according to WCCF those modules are slightly different in size than normal GDDR5 chips.
An earlier leak of the NVIDIA GP104-200 GPU suggests the GeForce GTX 1070 has 8GB GDDR5 memory clocked at 8000MHz.