I use RAID 0 but I installed it from scratch so whether you could set it up afterwards as you propose to do I really don't know. I've only ever had 1 RAID setup and that is on the PC I am on now. It does say in my manual that 2 identical NEW hard drives are required so I assume that means blank but as I said I don't know the answer to this.
Regarding the RAM make sure it will support it before purchase, Crucial can tell you this if the manuals not a lot of help.
I did consider Raptors as they are so fast but I did see a lot of people making a comment on them being a bit noisy and they are more pricey too. Perhaps someone with Raptors can comment on that.