That's a great querstion, it was actually one I was going to ask myself haha. What I suggest doing, which is what I plan on doing, is purchasing a 2.5 hard drive to replace the 20gig, if you do in fact have a 20 gig or plan on purchasing one. Simply for the fact that, even if you're not going to be running anything off the internal hard drive, 20 gigs can't hold too many demos or trailers. And for people like me that don't need all the stuff the $600 one comes with, it's cheaper to just upgrade the internal hard drive to a larger one, and still be under $600.
If you're looking for a internal hard drive to run linux and everything off of, i'd suggest one of these two-
Budget- 40gig SATA 2.5- $45.99 -
http://www.newegg.com/Product/Produc...82E16822116022
Higher Budget- 160gig SATA- $144.99
http://www.newegg.com/Product/Produc...82E16822148184
If you're looking to purchase a larger external hard drive I'd suggest one or all of these-
Budget- 160gig SATAII- $52.99-
http://www.newegg.com/Product/Produc...82E16822144415
Higher Budget- 320gig SATAII $99.99-
http://www.newegg.com/Product/Produc...82E16822136053
Now you can spend more then that, but those are just the hard drives that I feel are at great prices. The 160gig at $52.99 really is an awesome deal. $50 bucks for 160gigs is awesome.
And you can use any external enclosure you want. They range in price from $25+