I finally ordered a 4GB Apacer HT203 and installed it. I also installed the Vista USB patch. The hang problem is now fixed (see my earlier blog entries about ReadyBoost.) I can browse the thumb drive after resuming from sleep without difficulty.
However, ReadyBoost still doesn't work reliably after resuming from Sleep. After the computer wakes up, ReadyBoost works for a while, then mysteriously turns off (as seen by opening the Properties for the drive and looking at the ReadyBoost tab.) If I reenable ReadyBoost, it just turns off again almost immediately.
At this point, I'm only slightly better off than I was before I applied the Vista USB fix. The computer no longer hangs, but ReadyBoost still becomes disabled after resuming from sleep. Doing a "Safe Remove" and then reinserting the thumb drive, even into another port, doesn't help.