I am a fan of ReadyBoost and have seen good results using it with my work laptop. I tried something today and connected a second flash drive (2GB) and configured it for use with ReadyBoost. The first is an 8GB configured with 4GB for ReadyBoost. I'm using Windows 7 RC 7100 and was wondering if using more than one ReadyBoost drive helps?
Microsoft only recommends using "one to three times the amount of RAM" in your computer. Anything over that won't give you a measurable performance boost.
That aside, multiple Readyboost drives is a new trick in Windows 7. Measure your potential gain (baring in mind the 3x Ram rule) versus the added complexity and potential for failure of using a new tool.