Readyboost is really not that helpful, and it is often misunderstood.
It is simple enough. You take a suitably fast usb drive, and allocate up to 4gb of it to ready boost.
Windows will then look for small modules that it can load faster from a usb drive than it can from the underlying hard drive.
A usb drive has minimal access time, but the data transfer time is much longer than from a hard drive.
Vista will create a list of those modules that you use, and pre-load them on to the readyboost drive.
When you need such a module, it will fetch it from the ready boost drive. If the module is updated, it will be written to both places.
If you unplug the usb stick, the official data is still on the hard drive.
It is not that helpful because it improves the fetch time of small modules only, and they already took minimal time.
Still, it doesn't hurt.
It is NOT any kind of a substitute for ram or a paging device.