Any high speed USB stick of 16GB or lower capacity will suffice. Make sure it has amazing read and write speeds. You can use more than one USB stick at a time, but you cannot combine their capacity as one big memory box.
This will require the latest firmware update of your Xbox 360.
Consult the manual or the online videos available on how to format the USB stick for Xbox 360 usage. 512 MB of the USB stick will be used for DRM.