Readyboost is not without its limitations. RB will use USB 2.0 flashdiscs, Secure Digital Cards and compact Flash Cards.
As far as I understand it there are two types of RAM in memory sticks depending on which you get, slow and fast. It seems it will only work with the fast type.
If you connect such a device Vista will run a test on it to see if it meets the standards needed.
The standards are 2.5MB / sec throughout for 4 KB random reads and 1.75 MB / sec throughout for 512 KB random reads. It should also have at least 256MB available for RB cache.
It seems you can benefit from RB equal to approx 150% of system RAM so your RB max would be 1.5MB.