When I built this computer I bought a usb drive, not for readyboost but so that I would have a easy & fast way to flash my bios. After actually doing that I had no use for it until I need to update my bios for some reason so I just left it plugged in. It's a 4gb drive I set 3gb for readyboost.
Does it make a difference... I have no idea
but I figured I might as well put it to some use other than letting it sit and collect dust