Calculate video RAM quantity need in 3D... how?

I knew easily how to compute 2D need, but I have in my back of mind wanting to figure 3D numbers. Recently I read the relation of 3D to 64/128/256/512... is similar to 2D to 1/2/4/8 instead of using it like GFX cache. Anyhow here is my concern. I had only monitors going 1024x768 at max, I have no plan to upgrade... show more I knew easily how to compute 2D need, but I have in my back of mind wanting to figure 3D numbers. Recently I read the relation of 3D to 64/128/256/512... is similar to 2D to 1/2/4/8 instead of using it like GFX cache. Anyhow here is my concern.

I had only monitors going 1024x768 at max, I have no plan to upgrade those, is 128mb enough to use? Even for cards like 7800 or 6800.
Update: The 3D is Direct3D as in the DirectX games.
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