I recently ran into a late 2000's mac pro that will not display anything but everything will turn on and the fans will start spinning , I put the graphics card (Quadro 4000) into my desktop and it worked just fine. I know macs aren't so friendly with Nvidia so I threw a old AMD card I had in there and still nothing on screen. I concluded that graphics card isn't the issue so I went to changing the ram around to see if that would do anything still nothing. Could it be the CMOS battery that died? I would like to hear what possible solutions you have, Thanks!