Not all flash drives are readyboost enabled. Plus if the specs in your sig are right. You will not get any gain from using REadyBoost. In fact you would lose performance.
ReadyBoost is mainly for those with less than 1GB of RAM and at least double that on a flash drive. So you would need 2GB for every 1GB.
So you would need 8GB Flash card to even notice a slight performance gain with readyboost. But since you already have 4GB of RAM. You wont notice it at all.