I've been running Windows Vista Ultimate on my Dell Inspiron 6400 since early April, but it wasn't until about a week ago that I started using a ReadyBoost USB thumb drive. I don't have an hard numbers, but I can tell you the difference is HUGE. I gennerally run Outlook 2007, VS.NET 2005, IE7 and MS SQL Server database with Management Studio, and Virtual PC 2007. My VPC instance usually runs VS.NET 2003 and several instances of IE7. Needless to say, I need a good bit of horsepower. Switching between applications used to have a visible delay, now there is nothing. Compiling my applications in Visual Studio is also much more responsive.
If you are going to run Vista, I highly recommend you purchase a ReadyBoost device.