For this issue, the following three forum posts at InsanelyMac were very helpful:
- NVIDIA 9 Series Driver Installer v2
- G94(9600gt) is fully supported by new mbp drivers!, ci/qe, fan control
- Reply to second forum post about ASUS Geforce 9600 GT
I downloaded the driver (NVIDIA 9 Series Driver Installer v2) from the first mentioned forum post. Then I modified the info.plist of in the NVDANV50Hal.kext (after I installed the driver from the first forum post!) as described in the second mentioned forum post, but I'm not sure this would be really necessary. I also had to use the OSX86Studio to modify the GFX string as shown in the third mentioned forum post (see also the picture with the settings here).