"Vista Ready Upgrade" does not mean ReadyBoost-compatible
I bought two SD Cards today, each of which are 2GB in size (and each with a warning on the back that this means 2*10^6, not 2*2^30, bytes).
One of them is to go into my wife's pink camera, the other is to go into my laptop, as a ReadyBoost drive (aka spare memory).
Now, a ReadyBoost drive has to pass a couple of simple speed restrictions - so, every time you try to get a drive to work as a ReadyBoost drive, Windows Vista runs a little test to make sure that the drive is fast enough to give a little extra performance.
Occasionally, someone will say "how can I disable the test, and enable ReadyBoost on a drive that isn't so fast?" - the answer, quite frankly, is "well, duh, you can't!" Okay, so that's a little tactless - how about I borrow a trick from Jesper, and simply rephrase the question: "How can I make Windows add a component into my system's data flow that deliberately slows the system down?"
When you put it that way, the answer of "well, duh, you can't!" actually seems appropriate.
I ran the test on my two drives - actually, I waited for the hard-drive light to stop running (Vista was indexing some stuff, and the test doesn't work so accurately when Vista is busy), and then I ran the test.
The SD Card from SanDisk (whose primary selling point, apparently, is that it has a pink label) passed the test.
The SD Card from PNY, labeled "Windows Vista Ready Upgrade", failed the test. Over and over again. Really - I tried the test several times, and in between, I tried it on the other drive, which passed over and over again.
I played the odds, and won - one of the cards was suitable for ReadyBoost, and the other was suitable for the camera.
My suggestion to you is to ask the guy in the store - "Is this disk / card compatible with ReadyBoost?" - so that you can take it back for a refund if you find it fails the test.
Ironically, SanDisk's web site has a section on it talking about ReadyBoost - but they only describe their USB drives. I prefer my SD Card, which sticks out of my laptop only about a twentieth of an inch, rather than my USB drives, that stick an inch and a half out, and are always ready to be snapped off.