1. Gigabit Ethernet
2. Accessible over the web
3. RAID-5 Configuration
4. Built-in BitTorrent client
5. Support of terabyte drives
6. Media server (not required, but would really like it)
I'd like a box like this as well!
the trouble is I don't think that you'll find it.
my reasoning is simple, Raid5 is a business type feature, media server and bit torrent client is a home type feature.
What you might find is that if you get something like an overland snap server that runs guardian OS (a linux variant) that you might be able to install a pnp media server on it, and perhaps a bit torrent client.
(something like the snap 520 -which has dual GB network ports, web interface for connectivity, support smb shares, or ISCSI disks, has holes for 4 disks supports raid 5 and expansion through SAS shelves).
after that things get a little flakier.
I went the other way around than you did. I bought a linux based media player (emprex ME1) you can pick them up from ebay for cheap.
then I installed a copy of debian onto it.
so mow it supports, web server, FTP server torrent client, pnp media server (as well as playing media directly to the TV).
there is a disk inside the unit and I've got a USB disk plugged into the back, which means I can quite happily store things on the disk and just schedule a task with cron to copy my important data to the USB disk. not quite as good as RAID 5, but good enough for my purposes.
the bad side is I believe it only supoprts up to 750GB drives, (though may support larger ones with the latest firmware).
it'll support any size USB disk plugged into the back (if I remember correctly it supports reading NTFS formatted disks as well with the latest firmware).
and it's only got a 100MB ethernet port.
hopefully that'll give you some ideas for some good searches to help you find the product.
either a decent MAS device with an OS that you can actually interact with, -so you can customise for your purpose
or a media server with an OS that you can interact with -so you can customise for your purpose