I want to play Cod 4 on xbox live what is the cheapest hard drive can buy?

A month ago i bought a Xbox 360 Arcade the first game i purchased was Call Of Duty: Modern Warfare. I really enjoyed the game but i want to play it online and i also bought Grand Theft Auto 4 and i want to play it on Xbox Live as well. So my question is what do you think is the cheapest Hard Drive I can buy?