The 8800 cards have been trouble-prone. In particular, they get hot enough to cause the low temperature solder (used with surface-mount chips) problems. There are online procedures for baking them in your home oven to re-flow the low-temp solder. This sometimes buys you more time.
Many users have walked away from these cards and replaced them with Apple-firmware 5770 for about US$250. It is a substantial upgrade for not a huge amount of money, but it certainly is a pain that you have to consider this solution at all -- that the 8800 cards are not more reliable.
Thanks for the reply!
ok I´ve read a lot negative comments about the 8800-card...maybe I should try to bake it.
But do you think this is a graphic card related issue only, or do you think this problem could be related to other hardware?
If this is an obvious graphic card related problem, maybe it´s worth investing in a 5770...the 8800 is quite bad
The simplest test is to hold the Alt/Option key at Startup. This invokes the Startup Manager, whose code is all in ROM. it draws a blue or gray screen and then shows an Icon for each potentially-bootable Volume. If the screen stays black, you clearly have a graphics problem.
Whether you also have additional problems is very difficult to tell.
Yes the problem is solved!
I started by borrowing a friends graphics card and installed it in my Mac Pro, and both screens was working. I realized my 8800Gt was broken.
I baked the 8800GT in the oven, there is a lot of videos on youtube on how to do it, and it worked again!
But I was tired of my old 8800Gt which is slow and old, and I realized Nvidia has released drivers for Mac on almost every card. And the new OS 10.8 support by default almost all new Nvidia cards. So I bought a GTX670 card and installed, works perfectly!
I just bought a "off-the-shelf" standard model, Gigabyte GeForce GTX 670 OC 2GB RAM to be precise. Even though it requires one 6-pin and one 8-pin power cable (Mac Pro only support 6-pin) it´s not a problem, you can just use a 6-pin-to-8-pin adaptor. The power supply is well within limits, they put an 8-pin cable on the card if somebody wants to manually over clock the card, which is not a good idea if you got a Mac Pro, because then you´ll need to get an external power supply (the power from the Mac Pro is not sufficient).
I don´t get a startup screen, the displays remain black until the login screen. But that´s not a problem for me, if I want to boot on a different drive I just choose startup disk before restart.
So far, it works like a charm!
Not just OC'ing a card but some programs and games can push a card and force it to use more power.
There are people using a splitter to take one 6-pin and connect the two 6-pin. EVGA and others do have 670's that only require 2 x 6-pin while some cards use 1x6-pin and 1x8-pin connector. And some people want/need a card with more than 2GB for better performance, 2.5-4GB for CS6 and other programs.