I'm in the same situation here.
My 10.5.8 Server is running on the exact same MacPro 3,1 as Brian's, with the only difference being I'm running 6Gb of RAM, and I have a Hardware RAID setup...
I too need to keep all current users, groups, share points, permissions etc as I migrate to 10.8.2.
Is there anyone out there with some knowledge on the best way to perform this migration?
I'm almost tempted to come in one weekend and take the MacPro offline (after a TimeMachine backup), erase the disk and start new with a clean install of 10.8.2, then reset everything from scratch. It seems to me that this might be easier than progressing through a number of difficult (and expensive) upgrades.
Clone your old server.
Do a CLEAN install of 10.8, upgrade it to the latest.
Set it up like a new server.
If you want to keep your users/groups/permissions, you can exports users/groups and if you have to copy your sharepoints make sure you retain their permissions. You'll need to re-enter user passwords (an export/import of users doesn't retain passwords).
Resetup your sharepoint and vpn config from scratch. (nothing to it)
Its just not that much to setup the server from scratch. Why go through an upgrade/migration process when you have the opportunity to leave any gremlins behind.