Sources: AMD Has Created Navi FOR Sony’s PlayStation 5.

[HoloDust]

When Xbox 360 released with 512Mb of GDDR3, best PC cards from both AMD and nVidia had 512MB as well.
When PS4 released with 8GB of GDDR5, best PC cards had 4 and 6GB, respectively.

Somehow, I don't think anyone will be skimping on RAM in next gen as well.