Physically, the machines have to have enough drive bays for the drives. If you don't have 19 drive bays between the two machines, you would need to buy external bays to hold the disks. You might buy new drives; let me point out that you could replace 12 80 GB drives with one 1 TB drive.
You will also need enough hard drive controller ports. I have no idea how many drives your motherboards support, but trying to run ten in a case will probably require that you buy additional controller cards. Good ones will have more than one lane of PCI-E connectivity (or are P4 motherboards so old that you will have PCI or PCI-X?), otherwise the throughput is throttled by the PCI connection.
Finally, you will need an OS that presents the storage space as NAS. I've never done this myself, so I don't remember the name, but you will see a couple of them if you poke around on the forum.