You've probably not heard of it, or do not know how it works, i didn't until about 3 months back, read this article from http://blogs.msdn.com/tomarcher/archive/2006/06/02/615199.aspx
In human as possible, it makes it possible to put read-info-caches into drives outside the Operating System's (OS) drive so that your OS can read/write other things. The way its done, is secured (using AES--http://en.wikipedia.org/wiki/AES) and only stores read only data there. Its safe against possible drive-knockout (accidental removal) thus your data is safe too.
There's a pretty good standard requirements that you have to meet to enable/use Readyboost. So, make sure that USB drive (must be flash based) is fast enough for read/write operations.
And last note, if you already have a fast machine, forget this, enjoy your Vista!
From MS site:
Windows ReadyBoost introduces a new concept in add-on system memory. You can use nonvolatile flash memory devices, such as universal serial bus (USB) flash drives, to improve performance without having to add memory "under the hood." The flash memory device serves as an additional memory cache—that is, memory that the computer can access much more quickly than it can access data on the hard disk drive.