I changed the slot bracket on my HD3870 card from one slot to two slot today. The graphics card had previously received a two slot fan cooler and at that opportunity I did not change the slot bracket because there was none in the kit. I managed to grab a two slot bracket with a grill and exchanged the slot bracket. The Mac was shut down in a regular way. After re fitting the grafic card I tried to start the mac pro and there was absolutely no reaction when I pushed the power button. No light, no sound nothing at all. Fans are not starting and there is practically no reaction at all. I have tried all sorts of trouble shooting and used a different socket for the mains cable. I have exchanged the grafics card and tried to start without grafics card. Nothing! I have turned the machine over to see if a metallic object may have fallen inside and short circuited my Mac. Nothing loose is in there. I have a master/slave unit which switches all the peripherals with the Mac Pro. I can see from the change of LEDs that my Mac Pro takes power when I fit the power cable. So it should not be the mains cable, I would think. I have also eliminated the master/slave unit but nothing brings the Mac Pro back from death. Is there any advise what I can do beyond taking it to the AASP? What kind of defect causes such weird boot failures?