Flash memory drives are a convenient way to move files around, but in Vista they can also help speed up your system's performance. ReadyBoost can use your 512 MB-or-larger USB 2.0-based memory device as a high-speed cache. It also works with devices like CompactFlash or SD memory cards.
When you pop one of the aforementioned memory devices into a Vista system, one of the choices offered by the AutoPlay menu will be Speed up my System using Windows ReadyBoost. When you select it, you'll be taken to the device's Properties page where you can specify how much of the device's available storage you want to allocate to ReadyBoost (Vista will recommend an amount to use, and you don't necessarily need to use the device's entire capacity). You can safely remove the device at any time, since ReadyBoost doesn't use it for critical system data.