I have been using FlexRAID on my WHS server for over a year now and love it. It doesn't matter what you are using your computer for weather it be server or HTPC or whatever else. FlexRAID is easy flexible and you dont have to rebuild the entire system just to add a drive. I have 13 HD's totalling 27.5TB, all setup with FlexRAID, i currently run everything off 1 3TB PPU (Parity Drive) if any hard drive fails, that is my redundancy. I do not have a seperate backup, although i back up my OS drive, and my important files to an external drive in case of catastrophic failure. You can use an external drive for redundancy. FlexRAID can be setup in 2 modes, snapshot, or realtime. snapshot will update the parity drive based on the schedule you setup, or realtime is of course on demand update. I have found snapshot to be a better option for me as my data does not change often, and it is a bit faster than the realtime protection.
On to adding more storage; if you have run out of room inside your HTPC, you can ways upgrade to something larger. You may also want to look into a HBA or RAID card with external an external eSATA port or something like that. I attempted the same thing back before i had a server. but i found building my system into a server was a better option, external HD enclosures tend to be noisy in living rooms.
Hope this helps.