It looks like you're using a 32-bit X server on the 64-bit system
Does the problem go away if you boot with e.g. mem=2G?
Yes, the XServer started then.
Also after using your workaround by patching the driver everything worked fine -- even after reinstalling the second card.
Thanks a lot for your help!
Just a small question. Can you confirm that it is not possible to use TwinView and SLI at the same time?
If yes, is this a hardware limitation (e.g. there is only one compositing unit) or a lacking feature of the current drivers?