The least I'd recommend for ArmA2-optimizing storage-wise would be 2x 7200rpm HDDs in a RAID0 setup, since a single 7200rpm one isn't enough to prevent stuttering.

But yeah, Raptor drives, RAM drives or SSDs would be preferred, although all of them are quite costly compared to the storage space on 7200rpm HDDs.

If going for a Raptor I'd suggest the Raptor2 ones, since I suppose one could just as well run a RAID0-setup otherwise and get 5-10x the storage space of a Raptor1 disk with roughly the same performance (?).