Systems with 2GB Memory or less will see an improvement with ReadyBoost. You should dedicate a (approved) USB Flash Drive and just leave it plugged in ... in other words there is no need to use the "Safely Remove Hardware" icon in the system tray, even though it exists. This allows you to reboot or shut down the system and Windows will remember and set it up on the next restart.
You should make sure the above setting is enabled (Optimize for quick removal) ... you will find this setting buried in Device Manager, under Disk Drives, highlight your USB device and select: Properties > Policies
ReadyBoost is a nice new feature in Windows Vista, but is not a substitute for Memory it is simply "high-speed cache".
ReadyBoost Q&A (outdated but informative) | Boost Windows Vista system performance with ReadyBoost