Windows Vista and Windows 7 users can use USB flash drives to improve system performance by using the USB drive (or an SD card) as additional memory cache. When you connect a compatible removable storage device to your computer, Windows ReadyBoost will automatically launch and ask if you want to use the device to speed up performance with Windows ReadyBoost. (Don't worry, if you change your mind, you can later disable Windows ReadyBoost for the flash drive.)
The amount of space Microsoft recommends setting aside on your USB flash drive for ReadyBoost is one to three times the amount of memory on your computer; so if you have 1GB of RAM on your computer, use 1GB to 3GB on the flash drive for ReadyBoost.
Note however, that not all USB flash drives may be compatible with ReadyBoost. The drive needs to be at least 256MB and drives that have poor write and random read performance can fail the compatibility test. If you have a compatible device, though, using ReadyBoost can make a significant difference in how fast Windows starts up and loads applications.