quoted

I can not install more drives in the computer. It is a home theater 
computer in a small case. I was expecting to be able to let the raid 
manage the copies itself.

If the / was on a RAID5, would it be able to boot with 2 disks ?
If so, is it possible to convert my RAID0 to a RAID5 ?
For example, I could boot on a CD, backup / onto /home, delete the 
RAID0 array and recreate it as RAID5, then restore the backup. Would 
this work ?

Based on my testing (somewhat old now) and regular use, I would say 
raid10 is probably your best bet. It's fast and secure, and with the 
-f2 option for "far" copies it's able to give high transfer rates.

Doesn't RAID10 means RAID 1+0, which requires 4 disks ?