Question : Problem: what are the key differences between software and hardware RAID

This is kind of a generic question  but i am looking into the servers i currently have and am looking at purchasing another server to be used as a backup domain controller I am trying to kind of figure out what is currently on my DC and how and what the hardware is capable of,
The domain controller currently in use is a dell 1800 it has 4 Intel Xeion 3 GHZ processors
The disk drives are Dell Array SCSI Disk Drives
The IDE ATA/ATAPI controllers are INtel 82801 EB Ultra ATA Storage Controllers and the
SCSI and RAID Controllers are as follows
1. Adaptec Embedded Serial ATA HostRAID Contoller
2.DELL CERC SATA 1.5/6ch RAID Controller
3.LSI Logic 1020/1030 Ultra320 SCSI Adapter

Also i am looking at purchasing a rack mounted dell server to be used as a backup dc and for running some applications
Any suggestions on the type of hardware,processor, memory etc would be appreciated I am looking at the Dell rack mounted 860 server or something maybe a little more robust like the dell 1950 or 2950 servers
thanks in advance

Answer : Problem: what are the key differences between software and hardware RAID

RAID 5 requires 3 drives but can have more as long as it is an odd number of drives, sometimes people will have what is called a hot spare, it is a drive that sits in the aray doing nothing until another drive fails, than it goes to work and the array is rebuilt, the bad drive gets replaced and all is well.

Raid 0
 is a Striped set without parity. Provides improved performance and additional storage but no fault tolerance. Any disk failure destroys the array, which becomes more likely with more disks in the array. A single disk failure destroys the entire array because when data is written to a RAID 0 drive, the data is broken into fragments. The number of fragments is dictated by the number of disks in the drive.

RAID 1
Mirrored set without parity. Provides fault tolerance from disk errors and single disk failure. Increased read performance occurs when using a multi-threaded operating system that supports split seeks, very small performance reduction when writing. Array continues to operate so long as at least one drive is functioning.

RAID 3
Striped set with dedicated parity. This mechanism provides an improved performance and fault tolerance similar to RAID 5, but with a dedicated parity disk rather than rotated parity stripes. The single parity disk is a bottle-neck for writing since every write requires updating the parity data. One minor benefit is the dedicated parity disk allows the parity drive to fail and operation will continue without parity or performance penalty.

RAID 4
Identical to RAID 3 but does block-level striping instead of byte-level striping.

RAID 5
Striped set with distributed parity. Distributed parity requires all but one drive to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive.

RAID 6       Striped set with dual parity. Provides fault tolerance from two drive failures; array continues to operate with up to two failed drives. This makes larger RAID groups more practical, especially for high availability systems. This becomes increasingly important because large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt: the larger the drive, the longer the rebuild will take. With dual parity, it gives time to rebuild the array without the data being volatile while the failed drive is being recovered.