|The aux power connection is required by high performance gaming cards, not low end cards.The 8400GS is extremely low end, low performance & does NOT require an aux power connection. |
Do NOT disable the onboard video in Device Manager...you're only looking for trouble by doing so. Make sure your new card is fully seated in it's slot & that the monitor cable it connected to the new card. Boot up, enter the BIOS & see if there's a way to disable the onboard by changing one of the settings. Some boards will auto-disable the onboard when a card is detected. Try looking in the BIOS chapter of your motherboard manual.