ReadyBoost is a feature that was introduced in Windows Vista, allowing a flash memory device (such as a USB flash drive or SD card) to act as a memory cache. Windows 7 has improved the capabilities of ReadyBoost over Windows Vista, including: Maximum cache size has been increased from 4GB to 32GB. Note that to utilize a memory cache of greater than 4GB, the flash drive needs to be formatted with either a exFAT or NTFS file system. Support for up to 8 ReadyBoost devices simultaneously on the PC. Windows Vista only supported a single ReadyBoost device per PC. The ReadyBoost cache can be used during boot to improve startup performance, revealed.
More info: ReadyBoost