Clearing the cache is actually a function of the 360 and it's storage devices. Keep in mind, if you do this it will clear out every single game update you have ever downloaded. It will not delete your profile or gamesaves, but you will have to download all your game updates again... aside from PGR 4 if you want shadows back.
http://www.joystiq.com/2006/06/09/xbox-360-hdd-cache-clear-code-discovered/With PGR 4, remember... do not accept the update when you sign on if you are connected to Live. You will not be able to play online with PGR 4 if you decline the update.
Long story short, it's not worth it.
He who makes a beast of himself gets rid of the pain of being a man.