My 8800GT went kaputt (screen jagged up and crashes during boot under OS X) so I replaced it with GTX285, which only worked for 2 hours and then machine shut down and didn't power up again with 285 in. But I could still power up AND boot Windows with 8800GT, even if the screen is all jagged up. (Mac OS X doesn't boot, it crashes during boot with 8800GT). I take it to the apple store in France/Lyon and they tell me they think it's the logic board which has failed. They'll order one, and try, and if it works, I pay, if it doesn't I don't pay. I told them that the logic board couldn't have failed since the machine boots into Windows without any issues, and I can use the machine on Windows. It recognizes both CPU's and all the RAM, I even did a Geekbench test in Windows. What I suspected was Power Supply but they said Power Supply is fine. But checking power supply is a long procedure, almost takes an entire day to watch and record all the voltage inputs/outputs. It's not a simple task, which I highly doubt they did or even aware of. Now, my question is, Can a Mac Pro boot into Windows and be usable with a failed logic board? Have you ever heard of such a thing? I could even use that machine in Target Mode from my MBP. 99% a working Target Mode means all the I/O on the machine is working perfectly fine.