Yesterday I purchased a new 8GB Secure Digital card (only $21!) in order to free up my current 2GB SD card used in my Vista laptop for use in my new(ish) Canon SD1100 IS.
Curious as to what allocation unit size to choose when formatting for optimal Ready Boost, I read a number of blogs, The Vista Forums, Tom Archer's Blog, Notebook Review Forum and more. But as I browsed, I didn't seem to get the answer I was looking for, so I just experimented.
Most of the articles say to format the card as FAT, but I need encryption on the card to store backup data, so NTFS was needed. After the first formatting at the default size, 4096 bytes, it showed as unusable for Ready Boost. After a couple of tries, I found the winning size was the largest size, 64 kilobyte, which allowed Ready Boost to work and give me the encryption I require. If I get bored, maybe I'll try other allocation unit sizes and do a comparison; okay it'll never happen, I'm never bored.