generally speaking......
2gb vram is good for 1920x1080p monitor resolution , or 1080p
3 or 4gb of vram is good for 2560x1440 monitor resolution, or 1440p
4gb or more vram is good for 3480x2160, or 4K resolution
That seems to be a terrible guideline for the amount of recommended VRAM a person should consider. Correct me if I get something miscalculated......
1920 x 1080 x 32 bit =
66325200 bits / 8 bits =
8294400 Bytes / 1024 Bytes =
8100 KB / 1024 KB =
7.91 MB
3840 x 2160 x 32 bit =
265420800 bits / 8 bits =
33177600 Bytes / 1024 Bytes =
32400 KB / 1024 KB =
31.64 MB
Now I know these numbers get much bigger when introducing AA of varying sorts as well as double and triple buffering, but the amount of VRAM a game's textures takes up I think is a bigger factor in today's games. Max textures in GTAV or Shadows of Mordor requires 3+ GB of VRAM regardless of resolution.
You could argue that a 1080p monitor doesn't offer the fidelity to represent such detailed textures, but it would depend on the game and how it displays textures in the finished image.