With your RAM already upgraded to 4GB, I doubt that you will see any difference using Readyboost. Keep in mind, the flash drive is going to be limited by the speed of your USB port and whether the flash drive is USB 2.0 or 3.0. If your laptop is a few years old it may only have USB 2.0 ports. Check and see, and make sure to try it in a USB 3.0 port if you have one. I would still give it a go, just to see if I could tell the difference since you can pick up a flash drive for pretty cheap these days and they are handy to have around even it Readyboost doesn’t make a difference.
To try it, plug in a flash drive, then open up Computer, right click on the flash drive, select Properties and click the ReadyBoost tab. Autoplay may even pop up and give you the Readyboost option when you plug it in.