'''Without knowing what GPU/VGC you put in the box''' I can't answer your question exactly. '''Generally''' however "[http://support.apple.com/kb/ht3246|shared VRAM]'' is a more recent development and most likely not to be a part of your machine.
The VRAM is either on the card, or when not on the card comes from the machines RAM, only since about 08 has there been both/or switching of VRAM possible.
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