How can I enable the nvidia card in a system without a hardware-switch between the cards
Check out the project Bumblebee. I have the optimus technology and use Bumblebee + VirtualGL + bbswitch to make it work. It means you have to start the programs you want to use the nvidia card with a commandline tool called optirun.
Debian Bumblebee wiki
Bumblebee at github
Hope this helps you!