In my opinion you would want to disable all disk caching for the USB devices that you would be using for ReadyBoost. The reason is due to the fact that Windows would simply cache the data into RAM which essentially would defeat what you want to accomplish with ReadyBoost.
I have heard that if you have double the amount of USB memory as computer RAM with USB ReadyBoost that you can obtain significant results. The more USB memory you can provide, over and above the amount of RAM available, the better your results will be. However it is important to note that you will want to use good quality USB drives and not the SLOW OLD ones of yesteryear.
I'm getting ready to give it a try myself, and I'm prepared with a 3.2GHz duo core and 16GB of 1333MHz DDR3 RAM. I'm going to ReadyBoost mulitples of 16GB up to at least 128GB of USB memory. It should be interesting. (Then I'm going to add a Kingston 120GB HyperX 3K SSD as a caching drive just for kicks). I'm going to have a blast and I'll check those USB cache settings for you while I do it.