I'm thinking of buying an X-Box 360 but I'm confused about the difference between Arcade and Elite. It seems that the only difference is that Elite is black and also has a 120 GB HDD. So the only essential difference seems to be that Elite has a HDD as opposed to Arcade which has 512 MB built-in memory. What I'm wondering then is, what is the advantage of having a HDD? I mean, what can you do with it?