OK, I will explain in steps:
1. Install the new vid card wether it is AGP or PCI
2. Plug the monitor into the vid card not the old onboard slot
3. Turn the computer on (if no display, restart computer with monitor attached to the onboard slot
4. Once you are finally in and the computer is running, right click on "My Computer"
5. Click on "Hardware" and then "Device Manager"
6. Go under video adapters and right click on the Onboard video card and select "Disable".
7. Reboot your machine and plug the monitor into the new vid card if it is not already plugged in there.
8. That should do it. If not go back and and install all the drivers from the driver CD onto the computer and then disable the old vid card. To be safe you might want to do that first.
Just a little extra info, if the computer beeps more than once when you go to boot it up, then you are having hardware issues and the motherboard is not compatible with the vid card. I have had it happen so don't think that it doesn't.
If this doesn't work, e-mail me and I will help you personally.