Deciding on the right Flash drive for Vista’s ready boost should not be all that involved, but just in case, this guide below ought to be of some help.

If you’re reading this to see how fast today’s USB flash memory keys transfer data, then you should look elsewhere. This isn’t about moving large amounts of digital imagery or audio around. This is strictly about ReadyBoost.
What is ReadyBoost?

ReadyBoost is a technology built into Windows Vista that caches disk reads onto a flash memory device. It can work with USB memory keys, flash memory cards, such as SD and CompactFlash, and other types of flash devices. It caches all types of file reads, not just the working set, nor just DLL’s or other persistent operating system data…. Source: ExtremeTech