I tried to do this with a PIII 1Ghz, thinking that would be enough for a NAS, but I got very slow file transfer rates due to the old crappy hard drives. "Very" old PCs usually had ATA33 or ATA66 and two hard drives share that bandwidth on one bus making RAID essentially worthless.
I would say, not worth it.
For building a new NAS, I'd just go with some cheap low power kit, like a Celeron 530 or AMD A4 and then a case that fits plenty of hard drives. You can use something like openfiler or freeNAS to avoid an OS license.