alyoshka :
Yes, for all the questions.
500GB is perfectly fine....... I'd go for 300 If possible.....
Yes, you're right on the parity bit info too....
And yes, ONLY is the right word.....
Wow, completely incorrect.
RAID 0+1 and RAID 10 do not use parity at all.
Here's an intro to RAID:
http://en.wikipedia.org/wiki/RAID
RAID 5 (and, of the other commonly used schemes, RAID 6) use parity. Dedicated parity disks haven't been used in a long time (that's RAID 2, 3, and 4).
Rebuilding an array can be time-consuming as well. RAID 5 especially. I have seen online RAID 5 rebuilds take overnight.
Also RAID doesn't work at the "DOS level". It's either driven by the OS (typically only used on Linux/UNIX), the motherboard (what will happen in this case) or a separate hardware RAID controller.
Building the RAID array can take some time at first, but size of the drives doesn't matter nearly as much as where on the disk you place the OS. Traditionally, the quickest area of the disk is the middle, as the drive heads have to move the least amount of distance from any other point on the disk.
paulbelk, what is your goal for using RAID? I don't think it provides a lot of use for most home users.
RAID 0 will provide some additional speed loading programs, but not much else for the standard home user, and it introduces the possibility of losing all of your data. (Backups to an external drive is great to offset this problem, as long as the external drive has enough room. Newer motherboards should support booting from an external drive, older ones may not. In any case, it is likely to be quite slow.)
RAID 1 is great for providing data redundancy, but it doesn't provide much of a performance boost.
RAID 5 (3+ disks, any number) gives you some of the benefits of RAID 0 (fast reads) but also provides data redundancy. It is, however, slow to write.
RAID 10 (4+ disks, in even numbers) is expensive, doesn't make good use of your total capacity, but provides both redundancy and speed.
Chose 2 of the following 3: speed, data safety, cheap.
RAID 0 = speed, cheap
RAID 1 = data safety, cheap (relatively, no capacity benefit though)
RAID 5= data safety, cheap
RAID 10 = speed, data safety
RAID arrays don't usually fail in and of themselves, it's usually a drive that fails or doesn't respond to the controller (or motherboard) in the expected timeframe (this has been a noted problem with some drives recently, particularly WD Caviar Greens). With RAID 1 or RAID 5 this isn't a huge problem. With RAID 0 it's catastrophic.
Another reason for the standard home user not to use RAID is that the typical motherboard-based RAID is not portable...if you decide to move your drives to a new CPU/motherboard, the data is not going to come along for the ride.