In the meantime I tried nearly resource available here ony Tonymac's website to get my system running. After trying out about 15 different CDs (-ASUS, nv260, nv2xx, with or without DSDT, in preboot or/and in the root directory, with EFI-string and without, ... ).
I could really need some help over here ;-)
My system always shows the same behavior, namely blacking out the screen just before the installation appears (I uploaded a movie clip into this thread before, it shows that the last thing I see are white fonts on black ground, I didn't even get to the apple logo already). As this seems not to be uncommon for GTX260 cards, I'am still looking for a solution that works for me.
As said before, I am booting from an Image on an S-ATA harddrive, not the Retail-DVD. But i don't think this is the cause for my problems, because I just re-checked the functionality of this image over USB and FW800 on my MacBookPro which boots up just fine with it. I wonder if anybody else ever booted up from something else than the Retail-DVD ...?
So, what else can I try?