Adding system memory (typically referred to as RAM) is often the best way to improve a PC's performance, since more memory means more applications are ready to run without accessing the hard drive. However, upgrading memory can be difficult and costly, and some machines have limited memory expansion capabilities, making it impossible to add RAM.
Windows Vista introduces Windows ReadyBoost, a new concept in adding memory to a system. You can use non-volatile flash memory, such as that on a universal serial bus (USB) flash drive, to improve performance without having to add additional memory "under the hood."
Read the complete article at:
http://www.microsoft.com/windows/windows-vista/features/readyboost.aspx