Hi,
you technically can achieve your goal.
But you'll definitely be out from the standard configuration, and you need to have the requested skills to support it.
They are many way you can choose, I would say for instance
- SME installed on your 80GB RAID1 devices
- add a new RAID5 array from your 4 500GB disks (3+1 spare)
You only have to create the raid5 array and add it to your system, like if it was a new disk (look from point 3 at
http://wiki.contribs.org/AddExtraHardDisk ).
If you are not familiar with raid device management you'd better follow CharlieBrady advise and just have server install directly on the 4 500GB, will work out of the box without any other problems
G.