Article: http://www.ditii.com/blog/2007/04/14/windows-vista-superfetch-readyboost/
SuperFetch is an enhancement of the Prefetcher that you have probably seen mentioned in previous versions of Windows. The Prefetcher is in charge of storing program information so that often-used programs and processes can run faster. In Vista, the Prefetcher has been tweaked and changed to be much more aggressive and intelligent in its caching to give even greater performance. This new functionality is called SuperFetch...
...You may have noticed that Vista tends to use a much greater percentage of system RAM than on Windows XP...
...Many people tend to think of RAM as some sort of resource, and when it starts getting used up, they believe that they have a problem. In reality however, RAM is more like a cache. If your system is only using a small percentage of your cache is a huge waste. Imagine how your processor would work if its L2 cache never used more than 25% of its capacity. L2 cache is a small amount of high-speed memory that allows for high-speed access to the system's most commonly accessed data.
In previous versions of Windows, the Prefetcher wasn't very aggressive in populating RAM. With Windows Vista however, SuperFetch tries its best to use as much RAM as it can, because if you have it you might as well make use of it.