I have run into problems with NVIDIA 9600GT running as SLI. I was unable to get the GPU to work.
I had troubles with the nvidia-current building the modules since the kernel headers were not proper. For some reason, the linux-kernel-generic doesn't work and I have to specifically download the kernel headers for my kernel version using
After a reboot, everything was back to normal and the GPU driver worked fine.
sudo apt-get install linux-headers-`uname -r`
sudo dpkg-reconfigure nvidia-current
Hope it helps you too.