PLEASE HELP MOBO/GRAPHIC CARD PROBLEM

have mobo asus m2nbp-vm csm9 ( IT WORKS TRIED IN ANOTHER RIG) i have added pci express nvidia 6800(IT WORKS TRIED IN ANOTHER RIG)it will not load drivers or recognized this hardware ....checked devise manager its not listed i have tried to add new hardware it does show tried to add drivers for this still does not work... checked psu still does not work.....i have placed another graphics card 8400 gs/super reconized straight away .....op system is vista home 32 bit...psu is 500 watt disabled on board graphics chip still loads drivers for onboard nvidia graphics can any one help me to resolve this problem all the hardware has beed checked vista has been updated, bios has been updated , drivers have been updated, PLEASE HELP