I tried to reinstall the system by extracting the hard drive and reinstalling the system, but this did not change anything. So the only possible component could be the EFI firmware, which is responsible for the pre boot sequence, and hence it must have been damaged during an EFI upgrade, or is not compatible to my 3rd party ram, which worked perfectly in snow leopard. After some research I came to the point that I could not do anything but giving to a apple for repair, but I am still curious to understand, which component could have failed when upgrading.
But after a reboot it does not boot at all (no gray display, but noise from fan and hard drive) and does not react to any keyboard input (so no SMC/NVRAM/PRAM reset, no Target Mode no Recovery etc). Directly after the upgrade finished the machine ran smoothly. I recently upgraded my macbook pro 2008 (not unibody, running 10.6.8) to mountain lion.
