Just to test it out I used a 2 gig micro SD card with an adapter that I purchased to use for something else. The readyboost really works well. I just ordered a 4 gig SD card online for $17 to use as readyboost.

I looked into getting a thumb drive, but you have to buy one that is ready for readyboost. I also read bad reviews of one USB drive where it adds spyware and erases info when you update.