I will say this about RAID, its not really for the average home user. It can be good to have a realtime copy of your main HD, but it gets expensive and usually isnt worth it.
Purchase a new drive and just install it as a another drive, thats the route I would go. And just to clarify, I have been building my own PCs for years and have never used RAID yet. I use it all the time at work for very large companies though.