There's a new performance feature in Windows Vista that has piqued my curiosity. The feature is based on pre-fetching (where programs are loaded into memory or in an area on your disk before you run it, because the system knows you usually run the program).
There are some other enhancements to the pre-fetch feature, but the interesting one to me has been coined as ReadyBoost. This feature uses flash memory (like a USB drive or compact flash card), and loads the pre-fetch information on the memory. The concept is that this will make your system faster.
Knowing that USB drives are slower than hard drives, I found the claim curious. It turns out that the way they access the data must be in smaller chunks. While flash memory is slower, the access time is much faster. There are several assumptions that vary the analysis fairly significantly, but my most pessimistic assumptions say that any file smaller than 100k is better off on flash memory. My most optimistic assumptions say that any file smaller than 500k is better off. Over those sizes, a hard drive will be faster (assuming that you keep your disk defragmented).