Jim,
I've been using a SanDisk UltraII SD card with an IOGear SD to USB adaptor.
It works well for me unless I forget to tell Windows Defender to not run
against it.
If I let Windows Defender run then Vista re-detects the IOGear dvice and any
other USBSTOR.SYS enabled device connected and determines that they are
"Unknown Devices" and installs the "Unknown Device" driver for them so there
is a Caveat to using ReadyBoost.
I also have to say that I have yet to see a performance boost in production
applications. If I have to repeat a scene transition in a game it takes less
time to load up, but outside of that I haven't seen any kind of improvement.
To give you a feel, my machine is a 3.06GHz Northwood equiped 845G chipset
with a gig of 533MHz RAM and a pair of 250GB Barracuda drives in RAID 1.
For those unfamiliar with ReadyBoost the short description is that it
essentially moves the pagefile from the slower hard drives to a much faster
USB key. The pagefile is replicated on the HD still so that if the USB key is
accidently detached you don't lose uncommitted data, but Vista uses the USB
key as its primary pagefile location.