well you can use a 24-pin psu on a 20-pin mobo but you CANT use a 20-pin psu on a 24-pin mobo. how many watts is your power supply? try putting the old card back in, see if it works then, and then go into bios wit hthe old card, and make sure its set ok, like if theres a setting for "default video: pci-express" or something. yeah, how good is ur psu, maybe its not up to the job? are all the power cables firmly connected?