When I bought my 8 1TB disks the cost of them were twice of the cost Hitachi 2TB disks are today. In other words if I bought them today I would get 32TB. I would need a slightly larger raid controller 16 ports but not much price difference. A proper case, something like this. I put in a 24port internal card to be able to expand into all 20 slots in the chassis:

http://www.norcotek.com/item_detail.php?categoryid=1&modelno=RPC-4220
http://www.adaptec.com/en-US/products/Controllers/Hardware/sas/performance/SAS-52445/
http://www.hitachigst.com/portal/site/en/products/deskstar/7K2000/

Every RAID5 will contain 8 disks, not more to preserve performance and reliability. When the server is fully loaded it will have 3 RAID5 of 8+6+6 disks. BTW something like this will probably be my next NFS NAS server..

Now about power, when the disks are not in use the RAID card will allow them to power down..

DSP8000SE, DSP7200SEHC, DSP5200SE sides, DSP7200SE rears, 861v8, 210, Sim2 Ultranero 4, Stewart Cinecurve 2.35:1, XEIT CM-5E Anamorphic lens, Dune HD Duo Cinema 8K and Wireworld Platinum 48G HDMI -> UHD722 -> Shunyata Omega Ethernet. Shunyata/Furutech power-cables/filters. SR Master Fuse@861v8. Uptone Etherregen with 10MHz OCXO clock.