I have to admit to some confusion here (like that's a special occasion).
I have 8GB of memory installed, and apparently the system hardly ever uses more than a couple GB of that (OK, I got it on sale, and empty sockets bother me. What can I say, I'm a Guy.) So, I take it that ReadyBoost isn't going to help me. Why would the OS use the slower USB port than use the RAM sitting idle on a 800MHz bus?
However, I did get a nice 4GB USB stick that is fast enough for Vista to let me use it as ReadyBoost, so I tried it out, and sure enough it is flashing like billy-ho. So, apparently Vista is actually using it?
Any idea what gives here, or is this another MS mystery (where's Leonard Nimoy when you need him)?