cached where? do you have a dedicated graphic card, or are you working with the graphic card integrated into the Mobo, if its the last one then probably its asigning 3 GB of memory to your video...
It's good memory management. I'd have preferred it if it cached more. Don't worry, it gives it back when it is needed.
Its called superfetch. I did ask a question about turning this feature off on windows 7. Have a read Here. Simon.
Yeah, it's superfetch, it loads up frequently used programs into the RAM ready to open them up quicker. I disable the service for it personally.