I think other posters misunderstood you -- as I understand it, you have a legitimate 20GB DMS, which will work if you decide to do the hack. Whether or not you decide to build a bigger capacity DMS right now, you should patch your phatbox. If you do that, then if your current DMS fails you will just have to replace the hard drive inside it.
If you want to build a bigger DMS right away, get as big a hard drive as you can afford. The hot price point at the moment is for 80-100GB. You'll want a basic 2.5 drive, 5400rpm with 8mb buffer. The original drives in the DMS are Toshiba, so they fit just right, but you can get any brand to fit.