I have 4 HDs in an Antec Sonata II case. It is not loud at all. RAID 5 is the safest configuration for data since one disk can go down and no data is lost. It will also rebuild the lost disk once it is replaced. A good RAID 5 controller will also provide "hot" drive swapping and disk diagnostics that will warn of impending drive failures. The downside is that a good quality RAID 5 controller will likely cost as much as 4 hard drives. Cheap on-board or software RAID just don't provide the same data security.
For most people, an external USB drive and disk imaging software that performs daily incremental backups is a good option.
I have read many posts on forums from photographers who've been burned by the assumed security of a RAID0 or RAID! set up. If the controller goes ALL of their files go. Now this isn't a problem for gaming rigs whose purpose isn't moving and storing critical data.
RAID5 seems to be the way to go - and at considerable expense as you mention. I plan the alternate route (external HD and backup software) for now. Thanks for mentioning the lack of noise level on yours - no doubt due to the case you bought.