ExtremeTech have written a great article reviewing different USB flash devices and how they get on with ReadyBoost in Windows Vista.
"ReadyBoost is a technology built into Windows Vista that caches disk reads onto a flash memory device. It can work with USB memory keys, flash memory cards, such as SD and CompactFlash, and other types of flash devices. It caches all types of file reads, not just the working set, nor just DLL's or other persistent operating system data."
Basically it will boost the speed of your system.
They test a variety of devices all ranging in price and size. Their results are interesting and its exciting to see these devices available NOW ready for when Vista ships.
Check out the review here: http://www.extremetech.com/article2/0,1697,2017844,00.asp