Do you understand that to use the GFX card you must connect the monitor into it?
If you plug the monitor into the motherboard and the GFX card is still fitted then the onboard display of the motherboard is disabled.
Does your monitor have a DVI or HDMI input? if so, go and buy a DVI or HDMI cable and connect the monitor to the GFX card.
If the monitor is VGA/Dsub (the small blue one) only, then get a DVI to Vga adapter like in that picture.