Xbox Live is 5 years old this week and with all the new stuff arriving soon (Xbox originals, films and TV on demand, etc) it looks like we could all do with a bit more space.
This is the cheapest I could find so far.
The Xbox 360 120GB Hard Drive is the best option for media enthusiasts who game on Xbox 360. It is the largest storage option for Xbox 360. Expand your Xbox 360 experience with downloadable content. Save your game progress, Xbox LIVE gamer profile, game demos, and custom soundtracks from your own music collection. Download movies and television shows from Xbox LIVE Marketplace. Play original and retro games in Xbox LIVE Arcade.
The Xbox 360 120GB Hard Drive includes a data transfer kit to easily migrate all of your saved data from your 20GB HDD to your 120GB HDD. It comes preloaded with great HD game demos, videos, and an assortment of Xbox LIVE Arcade game trials.