This is kind of outdated due to advances in Video cards and Motherboards, but it should help someone.

#1
Read the documentation that came along with your motherboard first. Because the actual switch for disabling the onboard video could be a physical jumper OR a software BIOS setting, be sure you know the correct jumper (often labeled on the PCB, or how to change settings in your systems' BIOS.

#2
Install any devices and drivers for the other video card device, connecting any power molex cables to the power supply of your computer, and the most recent drivers downloaded from the manufacturer's website of your card.

#3
Plug in the monitor to the port located on the motherboard first, otherwise you may not get a picture at all.

#4
According to your documentation, find out if you need to open (or close) a jumper on the motherboard to disable the video, or if it is a BIOS Setting.

#5
Change the jumper (opening or closing it as needed) on the mainboard while the system is powered off and unplugged from the wall, or go into BIOS and change the Primary Video toto PCI/AGP based on the card.

#6
Often there is also a setting in BIOS on newer boards that allows the user to set either a PCI card or AGP card as the primary video device. Make sure this is set to your video card.

#7
Save Settings and Exit BIOS, if you had to change settings here.

#8
Power off the system entirely, unplugging it from the wall. Change your Monitor cable from the motherboard connector to the primary Display out on the back of the video card. This should be designated with a 0 or 1 in the event the card has dual-display capability.

#9
Turn on your system. If you receive a video signal, congratulations, you're done!

#10
If you do not receive a video signal, listen carefully for the normal beep that indicates a successful POST. (Power On Self Test). If there is one beep, turn the system off and plug the cable back into the motherboard display port. If you now receive a video signal, check your BIOS Settings or Jumper settings once again. If you still have difficulties, or you do not receive any signal on either port, then read your documentation on how to reset BIOS or change the jumper back to its original position.