System RAM vs VRAM

box o rocks

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Apr 9, 2012
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I realize that the more VRAM a video card has, the more system RAM is set aside for its usage (What's it used for? Screen buffer?). But is there anyway to determine how much system RAM is set aside per GB of VRAM?
How much more system RAM is needed for an 8GB card than for a 4GB card? 3GB card? 1GB card?