MacPro won't boot-up...I think it's hardware related

Sorry, this is detailed, but I figured I had to explain everything I've done so far and the issues I've had. Thanks much!

My MacPro won't boot up. Initially I thought it was a software problem, and went through this thread looking for help. After a while, I came to think it wasn't the OS but a hardware problem. Here's why:

The initial problem was that I couldn't login to OS X (Lion). It started with the screen flickering, and freezing up. One time I logged in, and there were these uniform dots across my screen, and a black border overlaying the screen. I restarted, and that was when I couldn't login at all. It would go the gray screen with the dark gray Apple logo. The sundial would appear below...and then suddenly freeze. At that point, nothing.

I tried to boot into Safe Mode, but this also didn't work. The same result.

I then decided I'd do a clean install from the Install DVD and overwrite my boot drive (everything is saved onto a RAID Array). However, I couldn't load the OS installer.

I then thought maybe it was the HDD itself (the boot drive), so I bought a new one and tried to format and install. Again, the same thing. The DVD drive would activate, the gray screen would appear, and then it would freeze at the sundial. But this time, the dark gray overlay would come on screen, with the small box that told me to hold down power key (the one in mutliple languages).

So I then began to suspect the problem was deeper. I ran the Apple Hardware Diagnostic test. (I disconnected everything except the keyboard and the display.) The quick version showed everything was fine, but the extended version came back with some varying results.

First, my specs.

This is a MacPro 3,1.

24 GB RAM (8GB installed by vendor, 16 GB recently installed by me)

Bay 1 - Unformatted new HDD

Bay 2 - Slice 1 of RAID Array

Bay 3 - Slice 2 of RAID Array

Bay 4 - Formatted HDD for Storage

When I ran the first test, it was as above. Here is the error message I received:

4MEM/62/40000005: 0x79600ff18

This made me think it was a memory issue. So I took out the new RAM I had recently installed and ran the test again. I got this:

4MEM/60/40000005: 0x79650d18

So I took the old RAM out and put the new RAM back in. I also put the original, formatted HDD with OS X back into Bay 1. Here's what I got this time:

4SNS/3/4000000: TMOP-128.000

The new kind of error led me to think the problem was the old HDD and old RAM (after all, when I used only the new RAM, I put the old boot drive back in). When I ran the test, with the unformatted HDD in Bay 1 and the new RAM, I got...no error message!

So I restarted my system, hoping I could load the Install DVD now and install Mac OS X on the new HDD in Bay 1. But...nothing. Same as the original result. Gray screen with a freeze on the sundial, and then gray overlay telling me to shut down. Aaargh!!!

Someone suggested trying to login to Single User mode. I was tried and was able to do that. But, what do I do from there?

So I rebooted, and then tried to login when holding down the option key. It came up with the option to load the Install DVD. I said yes, and of course, same result as before.

So, as a last step, I put my old HDD back in and started the system holding the option key. This disc is a dual boot drive with Windows 7 on another partition. It gave me the following options:

Mac OS X HDD (my Lion install)

Windows 7

Recovery HD

Install Mac OS X (DVD)

For kicks, I chose Windows 7 to see what would happen. It actually logged me in! ... but it was like I was in an old safe mode in this sense: the screen size was the smallest available (that is, all the icons were big).

This makes me think that maybe (again, maybe) the problem is the video card. (Nvidia GeForce 8800 GT) This might explain the flickering and freezing I initially had when I could actually login. But...really, I'm at a loss and don't know if I should take the time and money to get a new video card.

Any suggestions???

Thanks!

Naveen