Windows Vista has a new feature that allows you to use a compatible (more on this in a minute) USB flash drive as a cache for often used files.  This can speed up your system since it does not eat up your RAM to keep those files nor does it have to go back to the hard drive to fetch them.

This can be a major boost to those who cannot afford or their motherboard does not support, upgrading memory beyond 512 MB or 1 GB MB.

Compatibility seems to be a big issue though as the USB flash drive must meat Microsoft's specs and currently there is no logo certified drives.  Most of the time you just have to try them and see if they will work.  If you are buying one, you might want to make sure you can take it back if it does not work.

Determining if the drive is compatible is pretty easy, just plug in the drive and if it comes up with a window to configure Readboost, you are good to go.

Currently, ActiveWin is trying to keep a list of known compatible drives.  For that list and more information on ReadyBoost:

http://www.activewin.com/reviews/hardware/memory/vista/readyboost.shtml