Open the BIOS at POST, find the graphics selection and change it from on-board to PCIe, then save and exit the BIOS. Upon re-POST your display will be from the expansion card and not the on-board video.
I did change the primary video adapter to PCIE->PCI->IGP in the bios, but then it starts up with the onboard graphic card. I thought the mainboard doesn't recognize the graphic card but then after I installed driver to it, it still the same.
Okay so I did everything you instructed but the display adapter only show the intergrated VGA, it doesn't show the my new dedicated GPU which is a HD 6870. So the thing is now my PC displays without any kind of graphic card after I disable the integrated VGA.
My monitor has HDMI and VGA. The HDMI I plug into the HD 6870 and the VGA I plug into the onboard graphic. So what you mean is to plug my monitor to HD 6870 output, but then when I changed to HDMI mode the monitor receive no signal at all(?)