Ready boost (which utilizes superfetch) and pagefile are two seperate things...
PageFile: Let's say you have three programs open, but are focused on one; that one gets priority for RAM usage, while the rest of the programs load some of their memory resources into the pagefile or virtual memory. Gets swapped back and forth, depending on what you are doing.
ReadyBoost: ReadyBoost is nothing more than a way to manage superfetch data; superfetch is collected with or without ReadyBoost enabled, and with or without a pagefile. The reason to use ReadyBoost is because the data transfer speeds are quicker than the alternative: the alternative is it gets built as an index file on you system HDD. It works like this: let's say the first thing you always do on your PC is check your email. Superfetch will build a profile on each user, which will "learn" your habits; i.e., what programs you run before lunch, after lunch, weekdays, weekends, when do you do maintenance, etc... when you go to launch your email, superfetch will have "predicted" that you were going to do this... as such, all the critical files associated with checking said email will already be loaded on the thumbdrive, or stored on your HDD if you don't use ReadyBoost, to be shot into memory the minute you click to open it.
Another thing: compression. ReadyBoost enables you to store up to 8GB of compressed data onto a 4GB thumbdrive, provided that the thumbdrive meets the minimum specs of course. While you could move the slider to use all 8GB of a 8GB thumbdrive, you will really only use 4. Restart, and check for yourself... you will have 4GB free on the thumbdrive