If you have an xp PRO or windows 2000 pro OS, you can do a software raid by patching some files and glue the drives together. The info is from Tom's Hardware ~2004.
I did it on a networked box using a gaggle of three drives plus a boot drive. The drives are a 400g PATA, 400g SATA, and a 320g SATA. I used the 1st 80g of the 400g drives for a striped 160g drive and the remaining three 320g chunks for a raid 5 drive of 640g. (all numbers in decimal gigs)
I don't know what is available for software raid in the Linux world, but this windows approach will work fine. The OS $$ is the main drawback. Also the speeds can be comparable to hardware raid, since the bottleneck is usually the drives themselves.
Raid 5 is fast on the read, but slow random writing because a single random write requires at least 2 reads and 2 writes. However, file servers are mostly being read, not written to, so the redundancy and size efficiency of raid 5 will usually be more important.