Theoretically, you just stick a new 3.5" IDE HD in there and use the "restore to as new" option on the upgrade CD. Hold down the menu button while it's booting up to bring up the upgrade CD menu, of course.

Note that I've not tried this.

You're probably limited to 128Gb with the version of Linux on there, though. And you'll probably want to get a low-noise disk (the new Seagates are pretty quiet).