Check out: -
http://www.freenas.org
I had an old PC (P3, 500MHz, 128Mb ram) and dropped all my spare HDs in it.
Had a NAS up and running in about 25 minutes - Cost=£0; Storage=60Gb
Supports Raid 0,1 and 5 in software!
Of course, now having proved the concept, I am going out to get a set of matched 250Gb drives and will set them up as a RAID 1 mirror.
Regards,
Shuttlebus