Have a look at this little utility for monitoring your readyboost performance.
http://www.softpedia.com/get/System/System-Info/readyBoost-Monitor.shtml
or if you are into sidebar gadgets like me try this one out.
http://klinkby.wordpress.com/2007/05/27/readyboost-monitor-gadget/
Let us know how you get on.
scottbelton@tiscali.co.uk