Here you go this is an another feature in vista and windows 7. People using the system frequently know what this is, But many of them don't know how and why to use this. The basic thing is that you can use an USB pen drive as a RAM for your system.
This would reduce read/write operations and allow the user to observe an increase in his or her system's performance-as well as result in a little more "green".
I explain this taking an example: A user is using a single core pentium 4 processor, when you install vista in that PC it becomes really slow I can say that system can take nearly 5-10 mins or more to power on completely( i.e the time taken between the time you power on till the time processor come's to its regular burden to 2-5%). But, when you use ReadyBoost it took me 8 minutes to power ON my system(I've experimented this diffence myself as a technician). So it isn't necessary to take a new PC or buy a new RAM extention which makes a "Go Green".