Appears my late 2008 MacPro is maxed out OS-wise with El-Cap. it's a MacPro3,1 w/ a single quad core processor. am i right?

Yes officially El Capitan is the last supported OS for the 2008 Mac Pro.

But search the net for ' (insert name of OS here) unsupported macs'.

This is written on a MacPro 3.1 2008 with El Capitan, macOS Sierra, macOS Mojave

and macOS Catalina installed.

All run perfectly well, Mojave and Catalina need a 'metal' capable graphics card though.

'Metal' refers to the API that drives the card you can read more here if you like,

https://en.wikipedia.org/wiki/Metal_(API)

If you want to check that the graphics card you have installed is Metal then open System Information

(Applications/ Utilities) click on Graphics/ Displays, and see if Metal is listed, for example.

If you are considering buying a Metal graphics card then make sure it is compatible with the mac you are installing it in,

try and buy a Mac Edition card or 'flashed' for Mac, this will ensure it has the mac boot efi so that you get boot screens and

the StartUp Manager. You can use PC/ Windows cards they will work but you will not get boot screens or the StartUp Manager.

Upgrading to Sierra

Your Mac must meet the following requirements:

You must have at least 2GBs of RAM installed and at least

10GBs of free storage space. You can upgrade to Sierra from

Lion or later already installed. Also, see How to upgrade to macOS Sierra.

Please also see The MacOS Sierra Compatibility List.

The list of Sierra compatible Macs:

2010 or Later

MacBook Air

MacBook Pro

Mac mini

Mac Pro

Late 2009 or Later

MacBook

iMac

As you can see above, your MP must be from 2010 or later to upgrade past El Capitan.

Are my applications compatible? See App Compatibility Table - RoaringApps.

Metal refers to the latest video capabilities when using Mojave or Catalina. See Metal - Apple Developer and Possible List of Supported GPU's that are able to run MacOS ...