For systems without reliable power backup, raid 1 configurations (mirroring) are more reliable than raid 5 or raid 6. We have a UPS so we just use raid 5 and do regular backups. But with some of the big disks now available, you can get a 5 GB mirrored array with just 2 disks. Nested raid configurations can also be used, that is mirrored raid 5 or raid 6 arrays for faster read/writes.