If thats the case and you want to also have maximum performance then I would shoot for a small SSD. 36GB should be plenty from most operating systems. However this is of course contingent upon your budget, as a lightning fast drive can be had for as little as $120.
It is never a good idea to have your OS running on the same drive that is hosting files. This can result in bogging down the entire machine under heavy usage and also limit the maximum throughput capabilities.
I preorderd this unit and at this point for my intents and purposes I am very let down.
The interface is very attractive and snappy, and the software package is very good as it has command line telnet support right out of the box (or with the addition of a HDD should you not have purchased one...