For comparisons:
A "Fast" SD will read between 15 a 30MB/s, and not taking in account slower write times.
The internal Ram does something along the lines off 75 a 100 times that.
Like Klaus said,
The basic idea is to put a lot off small files ( the type that a HD is normally bad with, but a flash drive not ( low access time ), and use the flash drive to load those system files ). Basic idea, but in practice, most readyboost sticks, suck ass when it comes to speed. In other words, the gain is minimal at best. On your PC your better off with Linux + Lots off Ram ( 4GB, 8GB
), and using the OS its caching, resulting in more speed, then anything else ( and saves your HD from those horrible I/O requests ).
The Pandora with 256MB ram can run for example: OS ( Linux ), FireFox, Gimp, MP3 player, and still has 75MB free ( maybe more, i don't remember if it was "real" free memory, or without the cache that Linux does ).