It depends on what you want, and what you want to pay for, as well as your tolerance for a slightly higher power bill over the long run unless you can run the system with sleep-mode enabled such that recovery from sleep doesn't mess with your clock settings -- even the memory timings.
Just by its very nature, large volume disk storage is a bottleneck in the pyramid of system architecture. At the top, CPU registers are extremely fast, and store minute volumes of information. At the disk end, the devices are relatively slow, and store large quantities of information at a low price ratio.
It makes sense to widen the bottleneck, if bus speeds are ample. With RAID0, you get performance increase to the limit of double throughput, but the chances of hard-disk failure are proportional to the number of disks in the array, while performance increases with the number of disks, as does power consumption. If one disk fails, you lose all your data -- effectively, as stored on the entire array.
With RAID1, you get data reliability with some small increase in performance improvement. You consume more power in measure to the number of disks, and data storage capacity is confined to single-drive size.
There are combinations of RAID0 and RAID1, but they effectively multiply your power requirement because they use more disks, and the capacity of the array for storing data is limited to the RAID0 component.
With RAID5, you need at least 3 disks. Your storage capacity at that threshold is 2/3 of the total for all disks, for the increase in parity data about the data that is stored on any remaining two drives. Performance compared to RAID0 is somewhat degraded because of need to process the parity data. If you increase the size of a RAID5, you get proportional increases in both speed and capacity, but you increase the chance of two or more drives failing at once, and RAID5 is only tolerant of a single drive failure.
RAID6 -- check if I'm wrong -- is tolerant of a two-drive failure, and I think it needs at least four disks.
On the power-consumption end, some company -- can't remember but thought it was "ACORN" -- produced a NAS that employed various RAID configurations for 5,400 rpm notebook drives. These drives consume less power, and are smaller, so the proper configuration would allow you to do a round-robin on spares that can be stored in a fireproof safe as clones or bootable backup partitions of the array. And Western Digital told me that SATA 2.5" notebook drives are desktop-system-compatible, so you can also go that way . . . .
But ultimately, if you want an equally significant performance boost, you would want to spend $300 or more on a hardware controller with its own onboard processor/XOR-engine and at least 128MB of cache or buffer RAM.
I don't just assume my RAID5 has assured reliability; I back it up to USB drives or a networked-file-server RAID5 array. And I have hot-swap drive-caddies for backup of the server's RAID5.
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