This game requires the Xbox 360 hard drive or the 512 MB Memory Unit for storage. (marketplace.xbox.com)
If you continue to use an unauthorised memory unit after the update, you will not be able to access your stored profile or saved games.» (thesixthaxis.com)
There are pros and cons for each option: for instance, if you do plump for the Core System you will have to buy a separate memory unit if you want to save your games. (amazon.co.uk)