i'm trying to get 10 years out of it, which looks do-able, maybe. it refuses to install the latest OS X's though, so it may finish its life as a linux box.
will 2019 finally be the year of linux on the desktop???
1. Hack Sierra installer to bypass restrictions. 2. Use target disk mode on Mac Pro and another Mac that supports firewire 800 to install Sierra. 3. Clone internal disk to external. Do upgrade to Sierra on it, then clone back to internal. 4. Remove internal disk, plug into external USB case, install Sierra on it. Plug it back to Mac Pro.
In all 4 cases you must hack the /System/Library/CoreServices/PlatformSupport.plist to make it bootable on Mac Pro.
You need to do it for recovery partition as well, if you ever want to boot to recovery partition.