Maybe I missed something, but I only see one computer in the mix. If this is the case it could be easy to give you the protestion you are looking for easlily and cheaply. If you are thinking about upgrading the machine anyway consider a MB that has onboard RAID (like the Intel and other boards). You then setup a Raid Mirrored configuration between two drives. How I would do this is actually I would setup two raid pairs with 4 drives. THe 1st 2 drives say 320 to 500GB eSata drives (should be less than $100 each) for the OS and software. The other two drives would then be in there own mirrored pair for their data or shares as you called them. This would give them protection by having the 2nd copy, and availablility if one of the drives fails in either set the machine keeps going. Another added benefit is performance since you are no longer going over the network but through a PCI-Express bridge chip.
Just my two cents.