Can a USB Flashdrive/Memorystick be used as RAM?

I read on a few websites that a USB Flashdrive/Memorystick could be used as RAM to speed up a computers performance. Right clicking my computer, selecting properties, advanced, performance settings, advanced again, then Change under Virtual memory. Here select USB drive and set paging file to desired MB. I have tried different sizes to no noticable change. I am running Windows XP Professional and have limited built in RAM, like 704 MB of RAM.
The data transfer rate may be a problem even if it is possible.
What am Imissing here?
Thanx
BJ