as far as i know you cannot simply "install" or add RAID to a current hardrive. When i was thinking about it on mine i would have to format/reinstall and would lose all stored data.
Possibly this is a consideraition - unless you are thinking in terms of a new mahcine!

i think the two drives used should be identical.
What do you plan to do stripped - for speed OR mirrored - for protecting important data.