In hopes of extending its useful life, I just bought a 1TB SSD for my 2008 MacPro
It is a model 3.1 from late 2008. I use it daily as my business machine. It has not had any serious problems and stands up to any apps you can put on it. I even run Autodesk's Revit under Parallels/Win7.
The machine is as I bought it. No hot rodding. The two quad core Xeons still doing their thing.
I replaced some drives along the way, but that's it. It currently has two 2TB drives, one 1TB drive, and one 320GB drive that was the boot drive when I bought it.
I am booting now from the 1TB drive, using one of the 2TB drives for Time Machine. The other 2TB drive is for Carbon Copy Cloner backups that also make it a bootable backup drive.
The 320 will come out and the new SSD will go in its place.
I also just got two new 27" monitors.
The machine should be good until such time as Apple OSs will no longer support it. I'm guessing that will be in another two or three years.
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and you should have everything backed up on those drives when the day eventually does come to replace it. 