Tom's Hardware Guide has analyzed the new ReadyBoost and SuperFetch features in Windows Vista. Here's a short snippet:
ReadyBoost is meant to support the new SuperFetch feature by adding more memory to the system. Microsoft's intended storage device is a USB 2.0 Flash memory stick, mainly because these products are incredibly affordable and reasonably fast. Knowing that USB 2.0 memory sticks deliver between 5 MB/s and 30 MB/s you might wonder how this makes sense.
Most USB 2.0 Flash memory devices on the market offer a capacity of 512 MB to 4 GB. There are smaller and even larger products available; the mainstream is at around 1 GB. These storage devices are very popular, as they are durable and small, and they can be used as a key fob. Many users use USB Flash memory sticks today as they used to jockey floppy disks.