Can This Mac Pro Be Restored To Original State 2006 mac pro Intel core 2 duo, 4gb 667, Original GTX 7800 Nvidia WD(also a Nvidia 120 512mb), 500gb HDD, Memorex DVD Rom.

I got this free from a friend with no HDD therefore no OS. 
I purchased Snow Leopard from Apple Store near me along with a cheap HDD for a restore. I placed the new HDD into my own 2008 mac pro and formatted the drive GUID mac os journaled for fresh install on older model.

Placed new HDD in '06 along with Snow Leopard booted to chime held in C until the disc started to read, waited a few more seconds released C and left it.

The drive spun on and off for 15-20 so I started pressing the space bar, tab and enter at various times until it sounded like it was installing which it must have done because after an hour or so the welcome video played. I could hear it but no picture just like the full install period. 

Thinking it was the video card I purchased the Nvidia 120 512mb waited on delivery and installed on my '08 first and it worked so pulled it and installed in '06 hit power and nothing for a few minutes then the welcome video sounds again.

Repeated what I had done in the first install instance with the new card but the same thing occurred no picture at end of install just sound.

I have tried every start up option/ key press solution I could find to no avail.

I then came across hackintosh and found a boot disc that allows me to boot the hdd and when it loads the welcome video is visible but no sound. Upon opening 'About This Mac' it's apparent that the boot rom has changed as well as the Mac Version and as noob as I am I know it's not right.

The computer will strangely enough accept any x86 windows disc with full video from beginning to end of install, but I would prefer it mac.

Now I don't know how much of this is fact but I recall an article were a mac running dual boot OS X and windows with a VM on the windows side had somehow managed to completely scrub the first HDD containing the mac partition rendering it unable to boot mac os again.
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. :-)