It depends what you want your system to do for what type of RAID you want. There are 3 basic flavors of RAID.
RAID 0(Striping): This is where files to be written are broken apart and spread across the array. The minimum drives you can use is 2. It would then write half the file to one drive and half to another drive. If you use 4 drives it will split it up across all 4 and so forth. This type will give you considerable performace gains. Essentially, the load is split over two drives and the file writing process is quicker. I have been told that you don't gain any performance in reading files but I'm not exactly sure why yet. It is correct that if one drive breaks then you've lost information from both. Since half the file is useless you have to start over. So keep good back ups. It could also be argured that with a RAID 0 array is less stable, since you are now depending on two devices not to fail. Since either one failing causes a loss of all data, you are at more of a risk for data loss with a RAID 0 array. The benefits are that performace is faster and you still get all the space you paid for.
RAID 1(mirroring)- This type is pretty simple. Basically, everything written to one drive is copied onto another. Therefore, if either drive fails you have another backup ready to go. The obvious downside to this is that if you have two 100gig drives you only have 100gigs of storage.
RAID 5(striping w/ parity)- The minimum number of drives you can use with this type is 3. This set of drives acts as a striped array in that you get performance gains but only need one additional drive to be able to recover from any one drive failing. I'm not sure what kind of fairy dust they use but apparently it is possible. I'm pretty sure that if you put 3 100 gig drives in a RAID 5 array then you will have 200 gigs of space. Hence, this is more desirable than mirroring a striped array (0+1) since you are able to use more of the space you bought. You can make RAID 5 array's bigger than 3 disks but I'm not sure how much space is available as the numbers change.
Anyway, this has gotten rather long but that's how they break down. What kind you want really depends on your needs. If you really need all 400 gigs of space and not a lot of data security then you should go with RAID 0. If you do need security but not really all that space, then RAID 5 or 1 would probably be better.
Hope this cleared it up.
