with a failed HDD on a RAID controller, you do not format it or do anything with it. you plug it up to the RAID card and have the controller rebuild the RAID set. you will only see 80 GB of space, the difference in size with the newer drive will be lost. basically it will take the lowest common denominator (e.g. your 80 GB HDD). it is nice if your controller supports hotswap (i mostly work with servers) that way you don't have to even turn your computer off and the card will rebuild the set by itself on the fly.
if you are afraid of data loss, do what r_manic suggests, back it up.