One of the most mentioned features which Windows Vista has is the ability to use external flash memory as an additional cahce to speed up your system, aka. ReadyBoost, when I talk about ReadyBoost with people I always tell them that not every thumbdrive or memory out there would work as the link mentions.
So here's a quick low down of what I have used that is ReadyBoost capable. From top to bottom, left to right.
EDIT : One last thing that I need to mention... the media might have worked as a ReadyBoost source on my system.. but it might not work in yours... when in doubt, disconnect, and reconnect the drive a few times and see.
Kingston 4GB Data Traveler II Plus Migo - I had to format the stick to FAT32 and then run the readyboost test a few times, then only it detects as readyboost capable, and sometimes after my machine comes out of hibernation with the stick connected it doesn't detect as readyboost capable anymore. So I guess you'll just have to be a bit wary about using this as a ReadyBoost device. That said... do you know how cheap 4GB thumbdrives are now?? Bloody hell!!!
Kingston 2GB Data Traveler Mini - Haven't tried it with ReadyBoost enough to see how well it works, I can only say it does detect as ReadyBoost capable and that's all I can say about it.
Sony Microvault Tiny 1GB - This is what I put into my wife's old Fujitsu P5010, it's actually quite suitable as a ReadyBoost device, it's small, flat, easy to carry around with your notebook. Just one strange problem though.... every now the entire contents of the drive go missing, but the space is still shown as used.... and I have on idea why that's happening!
TwinMos 1GB Ultra-X SD Card - I use this in my Fujitsu P1610's SD card slot as it's ReadyBoost device, no problems so far, it works..
Kingston 1GB MicroSD Card :- When used in conjunction with either the InnoDisk USB MicroSD Reader or it's own MicroSD to SD Adapter the MicroSD card works well as a ReadyBoost device.