Better off getting something bigger this will fill up faster then you think it will from demos, game updates, and video game installs. That's not counting if you use videos and music on the hard drive also
a 20GB drive is really tiny these days. it's only 13 GB after all the system files. there's no reason not to spring for an unauthorized 250GB drive for $50 at this point (though the unauthorized drives can't play old games without modification).