If you have Vista max size is 4gb. If you have Win 7 then Microsoft would recommend up to 4 times your memory capacity which in this case would be 8gb. I have used the Sandisk Cruzer Blade models with good success. Very small and out of the way. Just check which ever one you buy to see if it is rated for Ready Boost. Some are too slow. http://www.sandisk.com/products/usb-flash-drives/cruzer-blade-usb-flash-drive
I would buy elsewhere as getting it direct from Sandisk is a bit expensive.