The "proper way to do it would be to:
- Back up the SQL database
- Export all your jobs
- back up the express share
And then rebuild the server with the new disks.
Another way would be to use your test DS to take an image of the prod DS and then drop that image on the prod DS with the new disks. You might need to install the HP Rapid Deployment Pack (RDP) or Dell/IBM equivalent on the test DS to image it.
The cowboy way would be to replace one of the 80GB disks with one of the 500s and let the array rebuild. then replace the other 80 GB disk and let the array rebuild again. Then use third party partitioning software to extend the partitions. I would definitely test this first and back everything up as above.
Some stuff doesn't back up well, like scripted installs.