The information and data that is held on your readyboost device is encrypted and backed up to your main hard drive, so therefore it is safe to remove at any time. After stopping the device using the icon (safley remove hardware) located in the notification area of your taskbar.
I have found the best gains are to be had on machines that are only running 1GB of RAM.
For more info on readyboost see previous forum post's
http://vistasquad.co.uk/forums/thread/346.aspx
scottbelton@tiscali.co.uk