Hey buddy........read this
Pretend the GPU(graphics processing unit) on a video card is the UPS man and RAM is a city or state. The objective of the UPS man is to deliver boxes to every single house.
Now.....If you only have one UPS man, would he be able to deliver a truck load of boxes in a city or a state faster?
So, the UPS man has the option to drive a 9600xt or a 5200. Because a car can only go so fast, he wont be able to deliver a truck load of boxes in the state of 256mb as fast as he could in the city of 128mb.
"He will deliver faster if there is only 128mb of houses. He will also get tired and sleepy on a road trip in the state of 256mb "
HOWEVER......If the UPS man happened to be flying a x800xl then he would be able to deliver his mail in the state of 256mb in an extremely short time.
ALSO, it is not practical to fly a x800xl within a city.
But because the UPS man is only flying a x800xl he wont be able to deliver throughout the galaxy of 512mb unless he had a spaceship powered by an r520 warp engine.
You will lose performance and pay extra money you dont have too.
Benchmarks....
http://www.guru3d.com/article/content/79/7/