A large case with 12, 5.25in drive bays that I can fill with several 5in3 SATA1/2/3 back-planes for hot-swapping drives. A couple of REALLY nice SATA cards that support RAID and hot-swapping drives. Then have maybe 24GB of Crucial Ballistix Smart Tracers @ DDR3 1600, put 20, 7200RPM 2TB hard drives into the hot-swap bays.
One cage would house only the active hot spares, all other drives would be in a very large Raid5 array, as I don't understand how to mix arrays just yet, as I have never had good controllers and enough drives to play with.
I would also have either a 10Gigabit switch, and a 10Gigabit NIC for network capabilities, OR a 4x1Gigabit NIC, and load balancing setup so the server could easily provide the whole home with anything desired with out huge issues, though the Raid5 array will potentially cause a bottle neck.
Case would be a generic low end case, as long as it supports 12 drive bays, and decent cooling capabilities to help pull air from the hard drives and out of the case. Those bays I like to use tend to let drives get a bit toasty.
Now, if I could have TWO of these machines, I would have one machine employ multiple Raid 0 arrays, while another machine uses Raid5, the second machine would be the backup location for the machine that has no redundancy, this way I don't loose 40TB worth of data.
This machine would be my server, for the entire home, storing every video/music file I wanted, I would have PS3 Media Server, and PLEX Laika installed on it, to enable all rooms to pull full HD content.