1. Yes, Gen3 cards backward compatible with Gen2 boards, excluding individual incompatibilities between exact models (right now nothing of such known about NVIDIA GTX600 series)
2. No, SLI connector useful only for joining of same models of videocard, i.e. GTX680+GTX680, GTX650+GTX650 and so on. There are third-party patches for NVIDIA driver to remove this restriction, and also software like Lucid to join different cards on application layer, but they are not errors-free. You also can delegate old GTX 465 board for CUDA/PhysX calculations, leaving new card for 3D.