There's been quite a bit of discussion as to whether ReadyBoost does anything for you if you already have 2GB of RAM. On my system, I'm running 2GB of RAM and a 4GB thumb drive for ReadyBoost, and I believe ReadyBoost makes a dramatic difference.
I'm hardly a typical user. Normally I have open: Outlook 2007, Visual Studio 2005, Virtual PC 2007 running Windows XP, and Internet Explorer with a dozen pages open. The Performance tab in Task Manager currently shows I have 1GB cached, which isn't much considering how much is running.
Earlier this week my system started churning. Everything was taking a lot longer than it had the day before. It took me a while, but I finally realized that the lights weren't flashing on the ReadyBoost drive anymore. I rebooted, reset the ReadyBoost drive, and performance returned.
So I don't have any concrete, measurable results that I can hold up as evidence, but the subjective experience is pretty clear. Many reviewers have tried to do synthetic benchmarks, such as loading five applications in a row. These tests don't do a good job of measuring what ReadyBoost is doing. Loading apps measures SuperFetch, not ReadyBoost. IMHO, ReadyBoost seems to shine when the system is under heavy memory load and you switch applications task.