I have the same board and my 8800GTS works fine after 'upgrading' from a 3870.
There are many things you can check and one of them could be your power supply. Maybe the card isn't getting enough power from it.
Make sure and uninstall all the ATi drivers as they might be interfering with the Nvidia card.
More info will be needed to help you further as everyone will be guessing what your set up is and simple through ideas at you.