Of particular interest for anyone considering ReadyBoost is the bookmarked Microsoft Windows Help and How-To topic which indicates that the recommended amount of memory to use for ReadyBoost acceleration is one to three times the amount of RAM installed. The example provided there is with a computer having 512 megabytes (MB) of RAM, plugging in a 4 gigabyte (GB) USB flash drive and setting aside from 512 MB to 1.5 GB of that drive will offer the best performance boost.
Click to find the bookmarks for Windows ReadyBoost.