SLI/CF doesn't double the memory:http://www.tomshardware.com/forum/245454-33-crossfire-faqs
"Do SLI or CrossFire double the memory ?
This is one the questions that many people make mistake understanding it .
The answer is NO , SLI or CrossFire doesnt double the memory , You have a 512MB card , adding another card wont make your memory 1GB, it still will be 512MB,so both cards will use 512MB of RAMs but the memory won't double.
Another example :
A game needs 512MB card to run at maximum settings , and you have a 256MB card , and you think if you add another 256MB card , your memory will be 512MB and thats not true.
You may see this mistake in many sites. "