If the drive is already failing, a RAID setup at this point would be a bad idea. Your first priority should be to backup all the important data you have.
Once you have done that, getting a replacement drive would be a good idea. and perhaps two discs for a mirrored array would be in order.
I do not know if the onboard raid controller for the P4P800 would let you take an existing drive and convert it to a mirror disk without losing any data, but it may be able to.