As I understand it, Vista will use these USB drives as expanded memory, (think RAM).
Quote:
Superfetch will monitor the data and applications accessed by the user in recent months and preload those into its memory. This allows for faster access to data and applications.
In current Windows versions the software loads applications and data only as the user asks for it. This takes time because Windows has to load not only the application itself after a system reboot but the drivers and other auxiliary applications.
"Superfetch works great if you have a reasonable amount of memory, and it works fantastic if you have boatloads of memory," Jim Allchin, group vice president for Windows platforms at Microsoft, told delegates in Los Angeles.
"But even if you don't have boatloads of memory, we have thought about that [with the USB option]."
Superfetch adds the memory on the USB key to the system's virtual memory, which in turn is used to preload applications and data which the user accesses frequently.
The USB option offers the ability to upgrade the system's memory even if there are no physical memory slots, allowing laptop users to increase system speed, according to Allchin.
Reference
The size drive you'll need is most likely dependent on your system and the programs you use most frequently. I would think that once Vista has gone retail and people are installing third party compatible software for it the Superfetch will require more memory.
For now, I wouldn't get too exited about it, but if you want to try it get the faster drive and see if it really makes a difference for you.