Show it off, of course. Ha.
Although I don't know if I could fill up that much space (I still have 184 GB left of my original 320...or 293), it would still be really nice knowing that I had all of that extra space. And it's probably going to be a long time before I get another computer. It took me maybe four months to fill up about half of my current HDD, so an extra drive of that size could be handy. 500 GB would probably be sufficient, but you never know.