Just a quick add-on, since despite this thread's age, it was one of the first search results I got looking for this info:
It may be better to use NTFS rather than exFAT, since NTFS supports encryption and compression, which exFAT does not. ReadyBoost uses 128-bit AES encryption to keep your files confidential should your ReadyBoost drive fall in the wrong hands, and also uses 2x compression, thus a 8 GB thumbdrive yields approx. 16 GB of ReadyBoost capacity. Not that anything in Windows really tells you that.