do you need any actual server functions? System backups? Web hosting? FTP?
It doesn’t seem so, so since you have the flexibility of the rack, i’d say just go big in there–find a nice big case with plenty of airflow, throw a powerful CPU and tons of RAM in there, and then connect all your HD’s and voila.
If you really want some extra speed, RAID is still an option with your HTPC, just be aware of the risks.
Oh, and make sure to take TONS of pics along the way! 😛
