From what I have read, my 2008 Mac Pro cannot be upgraded with a new CPU. But even if it could be, I'm not interested in spending the time/money to do so. I'm comfortable upgrading RAM, graphic cards, drives, but not the CPU. And since I use my computer to make money, I have no problem spending 4-5K for a new Mac Pro (and the necessary external storage) when the time comes.
The Logic board is not upgradeable. The 2009-2012 is X58 so you are limited to 1366-pin CPUs which are no longer being produced. They are still easy to get on the used market and perform well enough for many users (for the moment).
The 2008 and newer Mac Pros are able to run the latest OSX.
Read the first post in this thread for a new Mac Pro owner's perspective on older Mac Pro upgradeability.