ReadyBoost Question

Hi there.

I searched generally about this before posting but with no luck.

I have a new system with the new Core i7 920 CPU (don't know if this
matters :p) and with 6 GB DDR3 RAM (1333Mhz 6x1BG) running Vista
Business SP1 x64.

I intend to disable the pagefile completely and to use a dedicated USB
Flash Drive for ReadyBoost.

- What size should this USB Flash Drive have?
-I read somewhere that it should be a 2.5:1 ratio (in my case 15GB
then) for high-end systems. Is that right?
- Do I have to configure the device on every Windows Boot again, so
ReadyBoost is going to use it?

Thank you for taking the time to read it.

-Any answer will be appreciated and suggestions/advices if I should or
not do this as well. -