@OP: no you're not right.
no one is telling you to throw away your gfx card.
in case of cfx/sli, you get another gpu. the whole dual gfx card combo works like a dual gpu card with vram for each gpu. vram do not add up.
read this crossfire/sli faq:http://www.tomshardware.com/forum/245454-33-crossfire-faqs