RAM on a GFX card only does two things. It acts as a frame buffer and stores texture and object data for the processing units to manipulate.
The amount of RAM for a frame buffer is pretty minimal, even at extreme resolution. For a 32bit deep 1920x1080 display, even using triple-buffering, only about 25MB is needed for for framebuffering. The rest is used to hold data to be manipulated.
3D objects are actually quite small, being simply a mathematical representation of the shape of an object and information about its properties. The big use is textures, the images that get wrapped around objects to make them look solid and real. How much texture memory is needed depends on how many there are and how large each one is. There are games out there that will use several hundred MB for textures and there are other games that only use a few dozen. It all depends on the game.
Using multiple monitors does increase the amount of memory required since each display means another framebuffer but as I said before, each framebuffer is going to be less than 50MB, depending on resolution, color depth, and buffering levels.
I would be playing BF3 on high with a 1920x1080 resolution. I just need ed to know how much VRAM I need so I could play without it being laggy or anything else. Other places I looke said you would never use 1gb. Others said you need at least 1.5gb to play BF3 on high
According to my experience, You will have problem running BF3 with Ultra@1080p if you have 1gb vram.However, for high 1gb is alright.If you got 1.5gb and want to play it on high than it's perfect.With 1.5gb you can run BF3 @ultra as well.