iMac G3/400 motherboard (820-1051A). Replacement questions inside...

I recently bought a used Graphite iMac G3/400 DV SE. Long story short: I brought it home, installed more RAM (384MB total), confirmed firmware update had been installed (so I thought anyway), did a clean install of OS X (Panther) over OS 9. All seemed well for a few days of running Panther when all of a sudden the screen went dark. I tried every possible fix I could find on boards and nothing worked. I took it into a shop only to find out that the motherboard needed replacing. The shop's work order said something about the GPU detaching/overheating/melting...not sure exactly. The shop wanted $200 for a new/used motherboard and it wasn't even an equivalent board, lesser in fact. So, I decided to take it home and look into replacing the motherboard myself. My bro and I removed the old motherboard today and are about to make a purchase (ebay maybe) for another motherboard.
Now the questions...
1) Any thoughts on why the GPU fried and left me having to buy a new motherboard? The person I bought the computer from says that I should not have installed Panther, and had I left OS 9 as is that the computer would still be running. Thoughts?
2) The motherboard is model no. 820-1051A. Do I need to buy the exact same one, or will any motherboard do?
3) Once I install a new motherboard what are the steps involved in ensuring that OS X will run on the new board? With OS X installed on hard drive now, how will I now whether or not the firware update is installed on the new motherboard? Do I have to re-install OS 9, do firmware update, then try Panther install again? If so, how?

Any suggestions would be appreciated. I'm pretty nervous about this whole endeavor as I hate to blow any more money that I have to in getting this machine to run properly.

Thanks!!!