That depends on what you want. If you want software RAID, which is slower but requires no hardware (useful if you have run out of IRQ's or DMA channels), upgrade to Windows 2000. This giuves you RAID 0 and 1. If you want hardware RAID, you need to do some comparison shopping for a good RAID card that supports the type of RAID you want.
RAID 0 means that the data is split into two disks, but is half as reliable, for the failure of one drive destroys the data on both disks. RAID 1 eats up twice the disk space and maybe the disk bus bandwidth, because it creates an array of two identical hard drives. The benefit is therefore the downtime required to replace the bad disk drive. RAID 2 is an obsolete level of RAID and is no longer sold. RAID 3 requires three hard drives minimum, but doesn't eat up as much hard disk space. It provides adequate security and performance by striping data along two drives and putting corresponding mathematically generated parity data on the third drive. Parity data is used to mathematically reconstruct the data of a failed main HDD. RAID 3 has two drawbacks: write speed is directly proportional to the speed of the parity hard drive, and it requires all three disks to get involved in a write because parity dataq is computed on a byte level. RAID 4 is a performance improvement on RAID 3 but works in nearly the same way. Parity data is computed on an HDD block level, so it does not need to involve more than two drives if a single blick is written: the main HDD being written to and the parity drive. RAID 5 distributes the parity on all of the disks, so the minimum amount of drives is two drives.
One important note, RAID 3,4,and 5 add overhead to writes because the old parity and the old data must be read before replacing the old data in a write, because the parity's integrity must be maintained. There are four disk operations per write: read the old data, read the old parity, write the new data, and write new parity data that is computed from the old data, old parity, and the new parity. Second, parity data eats up 1/3 of your hard disk space.
All in all, RAID 5 gives the best performance if data integrity is a reasonable concern. RAID 1 provides maximum data integrity. This can be useful if you can wait for writes but cannot wait for drive rebuilds (i.e. a Web server that doesn't require much data logging). Another possible use of RAID 1 would be to store credit card ratings (a mistake in one of the drives can cause someone who has good credit to be unable to borrow money, or cause a bank to go bankrupt because it loaned a large sum of money to someone who obviously does not pay his bills and spends all the money that he gets.)
Compromise is the best word for choosing RAID. RAID 0 provides the best speed but halves the reliability of the array. RAID 1 provides maximum security, but is the slowest to write to. RAID 3,4, and 5 are compromises between speed and reliability.