How much space do you really need?
The biggest file hogs on my HTPC were .wtv files over 2 hours (NASCAR, F1, ALMS Races) and races from around the world (WTCC, BTCC, F1, FIA GT, FIA GT3, Blancpain, Aussie V8's and International GT Open) usually in h.264 or xvid hd.
However, Motorsport.tv has all this available for streaming now, no need to archive it locally.
Example. I used to have Pole Position on the HTPC, but since I have found it now on Hulu, deleted it.
What's on the server is basically anything TV Show, Animation or Movie that isn't currently in rotation on DirecTV (Hooper, Jayce and the Wheeled Warriors, Kolack: The Nightstalker). I plan on adding more older movies from the 70's, 80's and 90's, even some B&W Blurays. I will also continue down memory lane with 70's and 80's Anime including US properties animated in Korea or Japan (Transformers G1, Spider-man 1981, M.A.S.K, etc) plus newer stuff like Gundam 00 which is bomb!
I'll wait until I get a third 2TB/3TB drive before I rip Outlaw Star, SD Gundam, Zoids, Big O, etc.
Other big file hogs are PS1 games on average about 600MB per disc.
As I said, current free space on the 2nd drive stands at about 1.6TB. I will next move all my music files over to the server (45-50GB) and access them via Virtual DJ and only use one USB port instead of two. I have three tubs of CD's and my other to re-rip, likely to high bit MP3/WMA. This will likely grow to roughly 60-70GB.
The case I am going to purchase is this one - http://www.mitxpc.com/proddetail.asp?prod=6600&cat=185
While I would love a CFI Mini-ITX server case with four removable cages, it drastically bumps the price of my build up ($140).
This one holds 3 drives and one optical drive. Its likely I can get an adapter to put a drive in the optical drive space to give me (4) 3.5 drive slots.
I say use old hardware to get you off the ground and then upgrade to low power or depending on all you want to do (run Virtual Machines) low TDP, multi-core CPU.