It looks like you can use multiple flash drives for Readyboost. I am using a 4GB and a 2GB both are Readyboost enabled. Windows Vista would not let that happen. If you tried adding a new one, it took the new one and deleted the old one and vice versa.
I don't get what readyboost does, i tried it on my old 1gb USB and all it did was take up loads of space and i couldn't delete it... I'm not sure if I should try it on my 4gb or not, what does it do?
basically a 4GB readyboost flash drive is like an extra 4GB of memory. I'm running a 4 and 2 + 5gigs of ram, I can definitely notice a difference when I take them out.