It would all depend on next generation of consoles and their release year. I bought 3Gb card 4 years ago, when top cards from both manufacturers had 3 and 4 Gb

It would all depend on next generation of consoles and their release year. I bought 3Gb card 4 years ago, when top cards from both manufacturers had 3 and 4 Gb and it was an overkill. Now it seems that 8 Gb is a normal thing (same as PS4) and at least 4 is mandatory for AAA games.