i think you need identical hard drives for raid to work.. especially since they're different speeds i doubt it'll work.
there are different types of RAID setups for differnet purposes. For performance, RAID 0 is the popular choice. what that does is takes 2 identical hard drives and makes it appear as just one hard drive. when the system is writing to the HD, it writes a bit on each HD making the writing process shorter, and hence faster. if they're different sizes, the hard drive will be limited to the smaller one (e.g. if one is 80gb and the other is 120gb, it will read it as two 80gb hd's). But since your HDDs are different speeds, then raid 0 probably won't work for you.
I'm not sure why it would be unstable if you just have one as primary master and the other as primary slave.. it should be fine..
anyways i don't know much about this junk, so i'll let someone else jump in and help you out...
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius