I recently migrated my network storage server, running Ubuntu Linux incidently, to a software RAID 5 configuration. RAID level 5 requires atleast 3 harddrives; giving you in total N-1 storage, where N is the number of disks in the array.
For my deployment, I used 3 120GB drives (mixing IDE and SATA, Linux is indifferent here) giving me effectively 240GB. The missing 120GB is the parity information that allows transparent data recovery when 1 disk in the array fails. I’ve glossed over a lot of the details regarding RAID 5 but a quick google should fill in the blanks if need be.
Here is how I accomplished this under Ubuntu Dapper: