|ATX mboards are always being powered in some places, including some of the contacts in the PCI-E X16 slot, even when the computer is NOT running, as long as the power supply is receiving live AC power.|
Did you UNPLUG the computer, or otherwise switch off the AC power to the computer, at ALL times when you were plugging in and unplugging graphics cards ?
If you didn't, you may have damaged the PCI-E X16 slot circuits or the graphics cards or both.
Does the onboard video work fine when you DO NOT have a graphics card installed in the PCI-E X16 slot ?
If it doesn't, you probably have some other problem.
WHEN are you getting no video ?
Are you getting no video at all from a port on the graphics card while booting even BEFORE Windows is supposed to load ?,
OR - you ARE getting video BEFORE Windows is supposed to load, but NOT after Windows is supposed to load ?
(The latter is easy to fix, if there's nothing else wrong.)
What total (output) capacity does the power supply have ?
Minimum recommended power supply capacity for a system with one of these video chipsets on the card in the PCI-E X16 slot:
Geforce 8500 GT- 300 watts
Geforce 8800 GS- 400 watts
Geforce 8800 GTS or GTX - 450 watts
If the graphics card has one or two power sockets on it, you MUST connect a suitable connector from the power supply to that / those that fills the socket.
For when you're using the onboard video with no graphics card installed, and for when you are using a graphics card installed in the PCI-E X16 slot.......
Are you hearing the usual one short beep from the mboard while booting ?
OR - are you hearing no beep at all, or a pattern of beeps ?
If you hear a pattern of beeps, describe that e.g. number of them, short, long, do they stop or are they continuous.
Do the Power and Hard drive activity leds light up ?
Do you hear the fans and hard drive(s) spinning ?
"Is there something that I am not doing to enable these cards?"
Installing a working PCI-E X16 card in a working PCI-E X16 slot on a mboard there is nothing wrong with automatically disables the onboard video on your mboard.
The video chipset on the card, the mboard, and Windows all support the video chipset on the card (or the onboard video) producing basic video . You should always get video from the card while booting at least BEFORE Windows is supposed to load, if your power supply has at least the minimum recommended wattage capacity.
If your onboard video adapter is damaged - does not work when there is no card in the PCI-E X16 slot - even if there is nothing else wrong, when you install a card in the PCI-E X16 slot, the bios may NOT automically enable the video on the card in the slot.