Readyboost is NOT virtual RAM from a flash drive.
Readyboost is NOT RAM at all.
Readyboost is a cache used by your hard drive. Basically it copies stuff you normally use to the speedy Readyboost cache, where it can be used faster. Readyboost give an all round performance boost, and is more noticeable with older, slower hard drives. The more you have, and the faster it is means more boost.
Readyboost yields the most with small file browsing, boot up time, windows operations, and some programs you use.
Just wanted to clarify some things.
I have 20GB of readyboost, I got that instead of upgrading my HDD to a SSD or hybrid.