So I popped the thumb drive in, formatted it to NTFS (anything else and the RB file is limited to 4GB).
Mushkin tech support says I should use exFAT.
exFAT allows me to have a 32GB Readyboost drive.
From the article:
FAT32 has a file size limit of 4GB, while NTFS and exFAT are designed to support 16 EB (16.000.000 Terabyte).
NTFS is faster than exFAT, especially on Hard Drives. exFAT is an incremental improvement over the old FAT32 filesystem, and has a simple file storage algorithm which performs well on Solid State Drives, but not Hard Drives.