AMD is said to be working on a 8GB model as it saw 4GB as too much of a limitation for 4K gaming on such a high-end card. The performance is claimed to be about 30 percent higher than that offered by the GeForce GTX 980.
Unfortunately, it looks like the current launch date for the 8GB Fiji HBM card is early summer 2015 so we'll still have to wait a few more months to see how this shakes up the graphics card market.
Nvidia’s HBM High Bandwidth Memory card is coming in 2016 with the upcoming 16nm Pascal GPU and the GK200 that powers Titan X should hold the ground against Fiji. We still expect a quite high price for Titan X, as Nvidia traditionally prices this card close to $1000 and our educated guess is that Fiji might end up much closer to the Geforce GTX 980 in terms of pricing. AMD Radeon R9 390X is the possible name for Fiji, but we don’t have this part confirmed.