Remove power to the board. Remove the battery. Reset the CMOS. wait 15 minutes. Put the battery back in. Put only 1 memory module in the memory slots, keyboard, and video as you did before and see if it boots.

The good news is that the final code that this motherboard displays is 7F.  It means that the motherboard has completed POST but it needs some kind of user interaction to go forward.  What this usually is is a notification that the bios has been reset and you need to edit bios settings.  If when you get to 7F on the code reader, if you push F1 on the keyboard, it should go past to a FF (booted).  The fun part of this is that you have no functional display and that is what needs to be attacked.
 
Lets start with some basics.  What exact hardware are you installing in the base configuration that RJ is recommending. (BTW great idea RJ, I completely concur).  Please give brands and model numbers of the CPU, memory (and how many pieces), PSU, GPU, keyboard and mouse.  Please also describe what slots on the motherboard you are connecting the memory and GPU.  Finally please describe the surface that the motherboard is resting on for these tests.  Is it in the case?  Is it on a cardboard surface?  Is it on your kitchen table?  The more specific the better.
 
Second lets get a little more detail about how the codes are going through on the motherboard.  From the title of your thread you say it gives errors 26 and 7F.  Please describe the beep you hear when 26 comes by if any.  Does it sound longer and louder than the typical single beep you hear when a computer starts?  The 26 error is typically an error pointing directly at the graphics card.  It may not mean that the card is bad but that there is a problem with the slot on the board, but that is still speculative without more detail.
 
Hopefully with this information we can help you get the board chugging along.  The 680i was a great board in its day and still can run a good system depending on your needs.

Oh and hitting F1 does open the BIOS, and the display moves to FF - only I can't see anything on-screen. I'm using an LG monitor in analog mode with a convertor on the DVI end of the card. Not sure if I mentioned this, but the 1st time I tried the basic setup, I got to the windows logon screen (after loading default BIOS - perhaps foolishly…) but the system hung and I had to restart.
 

chadlebowski
Ah - one more thing - the additional graphics molex had a bit of resistance when being plugged in… is there a risk that the board bent too much?

The 8800GTS typically did not have a molex power connector (female version of the link).  It had a 6 pin PCIE power connection.  Is this what you connected?
Now there are adapters from two 4 pin molex power connectors to one 6 pin PCIE power connection and if is what you are using that is fine as long as you are connection BOTH molex connections to the adapter.
There usually isn't much rick of bending these pins unless you really shoved it in there.
 
Also you mentioned in your previous posts that you have tried a backup graphics card.  What kind of card is that?

chadlebowski
I meant the plug on the motherboard, and mean also bending the board rather than the pins. And the replacement card is the same, and known to be working!

It shouldnt, but it can.  Take a flash light and look inside the slot closely.  Are all the pins in the slot uniform?
 
Also, which slot are you connecting the graphics card to?  It should be the slot closest to the CPU.

chadlebowski
At work at the moment, will examine pins this evening… Also, GPU is going into slot closest to CPU. I've tried the blue slot too, and the second SLI slot, which gives a 5D reading on the mobo

This is normal.  One of the drawbacks to the NForce series motherboards around this time was that if there was 1 graphics card, it HAD TO BE in the 1st slot.  So putting 1 GPU in the second slot alone would cause this.
 
Looking forward to hearing back about the pins in the PCIE slot.  Fingers crossed that they are ok.

chadlebowski
Right - pins all seem fine, still no luck. Is it safe to assume the motherboard is finished?

Possible, but let me confirm one thing that seems to have been grazed past.  Are you connecting power directly to the graphics card?  There should be a 6 pin PCIE power cable connected to the graphics card.

Probably game over... but just for kicks, since you are going to dismount it from the case anyway, try to post the motherboard outside the case.  Place the board on a non-conductive surface like a cardboard box and connect just the PSU, 1 piece of memory and the graphics card.  Do not connect hard drives, keyboards, mice any kind of front panel cables.  Then use a screw driver to jump the 2 pins for the power button.  See if that does anything different.  Its not likely at all being that you have already tried checked for grounding, but you never know.

chadlebowski
Yeah the board is out of the case on a wooden surface. If I get the 26 long beep error without the graphics card inserted canI rule out the card being the problem?

More than likely yes.
 

And how exactly do I short the pins?

Take a screw driver or something conductive and complete the circuit between the 2 pins.
 

The board has a button, or do you mean where the case switch plugs in?

You can also use the button on the board.  I am just use to doing it the old school way. Both ways should work.
 

And do I have the psu plugged in when doing this?

Ohh yes... This is one last attempt to power on the board successfully.  I doubt it will work, but if it does, then you will have brought the dieing patient back when hope was fading.

Try booting with only one ram stick in the first black slot, boot and go right into the bios if possible and manually setup your voltages and timings for the ram and the voltage for the CPU. The auto settings on the old nforce boards usually would just crash them out. While in the bios disable legacy usb too. Then save and reboot and see where that takes ya.