Two plain ol' SATA hard drives. The motherboard says it has RAID, so I'm guessing that means it comes with RAID software, ect already.
Also, there are all kinds of RAID setups. One way will "stripe," meaning if you write a 200kb file, the first 100kb is written to HD-A while the second 100kb is written to HD-B. That doubles the transfer speed, but if one drive dies, the other has to be formatted because it only holds half of every file.
The type of RAID setup is chosen during setup; no matter which one you choose, all you need is two SATA hard drives and your motherboard.
EDIT1: If you do a mirrored one (the first one I talked about) and use, say, two 100GB hard drives, you'll still only have 100GB of storage. The rest of them give you the total, ie 200GB, but they improve something else (like the data transfer speed.)
EDIT2: The second sentence doesn't mean you HAVE to use RAID either; it's an option, though.