Just to explain it a bit. What happens is you buy a card like a 5970 which has 2 GPU's on board. This is to all intents and purposes the same situation as far as memory goes as running 2 cards in Crossfire.
Each GPU or card has to have a copy of the same information in it as the other one, so they both know where they are up to.
This means that if you have a 1GB card and add another one you still only actually have 1GB of ram available as by the time the memory on card 1 is full card 2 has an exact copy as well.