Can This Mac Pro Be Restored To Original State
There are several disturbing things from your screenshot I want to explain to you.
My answer is no direct answer to your problem, but in the end could help you, if you are willing to do some hardware modifications.

I just want to summarize your hardware description first and put my 5c to it.


First, it seems you don't have an original MacPro from 2006. The original MacPro has the model specification MacPro 1,1. Second, mine at home which was the first model introduced at WWDC back in time has been delivered with two dual core Xeons with 2.66GHz each in it, bus speed is 1.33GHz, RAM modules with at least 667MHz. Frankly, I never heard of a Core2Duo CPU in a MacPro. (see also http://apple-history.com/mac_pro)
The MacPro model 3,1 would be a MacPro Early 2008, so the guy you got the box from has made several modifications to it (to be polite and not to say "messed up"), including swapping the CPU's and installing a MacPro 3,1 firmware.
The MacPro 3,1 has 800MHz RAM modules and a bus speed of 1.6GHz and starts with two QuadCore 2.8GHz CPU's; additionally it is the first model with an 64bit EFI. (There is a BTO edition with only one 2.8GHz QuadCore CPU; my second MacPro under my desk which I have tuned to the max in the meantime.)
And that is the reason, why the 3,1 firmware on your hardware 1,1 MacPro is totally wrong - the hardware specifications do not match in any way with the MacPro 3,1 firmware.
The MacPro from 2007 (Model ID 2,1) is just a hardware release with higher speed CPU's, but is hardware identical (motherboard) to the original MacPro 1,1. So with installing the 2,1 firmware on a MacPro 1,1 it becomes a 2,1 with the capability to run faster clocked CPU's. (That is what I am doing currently with my MacPro 1,1.)
MacPro 1,1 from 2006 including model 2,1 has a 32bit EFI which limits the OS you can install on to Mavericks natively. You can't change that. There are several tricks to the OS you can do for running Yosemite on it (swapping some libraries emulating a 64bit EFI).
The best native GPU you could buy at 2006 for a MacPro 1,1 was the ATI X1900XT. (Still have that in my hardware shelf wrapped in anti-static plastic).


What you can do to have fun again with that box:


Downgrade the firmware to 2,1 (You need that for a CPU Upgrade to 3.0GHz Xeon CPU's, if you want this.)
Try again your Snow Leopard installation and upgrade it to Lion (10.7.5) then.


OR:


Downgrade the Firmware to 2,1
Buy a CPU upgrade to the maximum possible on ebay to two 3.0GhZ
Xeons. 
Install the CPU's (one ebay seller even provides an
installation video for it; contact me for the link.)
Install Mavericks.


I currently need to suppress the urge to book the next flight to you, fixing that good old manually sickened MacPro. :-)