You setup RAID on multiple identical drives, you don't just put in one drive and RAID it. If you use RAID1 then you Mirror the drives, losing 1/2 the space. One drive just has the same files as the other. If you use RAID-0 all data is spread across 2 drives as if they were one, if you lose one drive due to hardware failure you lose all data on both drives.
If you were to put in one drive and then add a second one later, and then put RAID-0 on it, you would lose all data on the first drive in the RAID set as doing the RAID configuration wipes the drives as it is configured. You need to plan this out so you have 2 identical drives and RAID them all at once.
I prefer RAID-5 where you need 3 identical drives, all data is spread across 3 drives and if one dies you lose no data and can replace the drive and get back into a RAID 5 configuration without any data loss.
When I built my system, I purchased a 120GB SATA HD. I would like to purchase an addition identical HD to set up in either RAID 0 or 1. I see that a lot of the manufactures are setting there new systems up like this with an additional IDE HD. My questions are what are the pros and cons of RAID 0 and RAID 1. If I were to get a 3rd IDE HD to store digital content(Pictures and Video), and I was doing editing on these items with programs that are on the SATA Drives, would I still see the benefits of the RAID setup?