Windows Vista contains a performance booster functionality called ReadyBoost which really can improve the performance of your Windows Vista system. In fact, ReadyBoost, is one of the top five Windows Vista features according to Ed Bott.
In short ReadyBoost caches programs on a flash drive to speed up your Vista, reports are that a program that previously started in 10 seconds will now start in 2-4 seconds.
Almost all articles I have read on this subject says that you should use an USB-stick as your ReadyBoost flash drive, but it will work with any kind of flash drive, like an SD-card.