Someone with Steam experience please speak up
What we really need is someone with Steam experience to speak up and tell us what card they use. I can only speak theoretically. I know through a friend that GT610s work fine (Xubuntu 12.10) for normal desktop work and multi-monitor works with the Nvidia driver. Personally I run HD2000 graphics - but have no interest in games. I do not think KMS is really a problem once the Nvidia driver loads and you reach the log in screen. Unless there is some other issue with Steam and Wine. Is KMS really an issue that matters?
Putting an Nvidia card on a motherboard with integrated Intel is not the same thing as Hybrid graphics. In your case the Intel will no longer exist (to the system) once you plug in the Nvidia card and Bumblebee is not relevant.