Have you tried a safe boot ? VNC ?
I tried booting without a GPU card (headless):
The machine seems to boot (I hear the chime), but I cannot ping it, see shared folders via Bonjour, or Screen Share. No VNC. Same results from safe boot.
I tried resetting the PRAM:
No change in outcome.
I tried booting with the new GTX650 card (headless):
No change in outcome.
I tried booting with the new GTX650 card (w/ DVI monitor):
No change in outcome.
I tried resetting the PRAM again:
No change in outcome.
Note that I am using 10.8.2, which appears to have issues with being "headless" (or in this case, GPU-less).
https://discussions.apple.com/thread/4151334?start=30&tstart=0
https://discussions.apple.com/thread/4158931?start=0&tstart=0
http://www.avsforum.com/t/1421779/mountain-lion-and-mac-htpc
I tried booting with the original 5770 card (headless):
No change in outcome.
I tried booting with the original 5770 card (w/ 1 or 2 DVI monitors):
All is normal.
The only way I can get this machine to boot AND communicate is with the original 5770 card.
I realize now that ping, file sharing, screen sharing, etc, don't work until I'm logged in, so those were not useful tests I performed earlier. Sigh.
If I assume that the computer boots correctly when it's headless, it should pause at the login screen. I should be able to press the right arrow, Enter, type my password, press Enter, and be logged in. Doing this from a headless boot has no apparent effect on the machine.
Blah, planning to return this card and get one that is known to work out of the box with ML.