Late reply. But I can understand why you did not get any replies. It's hard to understand.
Assuming you plugged the monitor into the right card (the N275GTX). You may have to specify in the BIOS to NOT use the onboard card, but the PCI Express card.
Then, yes, you can uninstall the onboard card's drivers, look for the intel or dell drivers. If you cannot find it, look for it under device manager, you should see it there and can uninstall it there.
Then if still no luck, make sure you have the correct drivers and the latest drivers for that card. Those that come in the box with the card is not the latest. Download the latest from nVidias website.
If you run that, it will install, even if the old drivers are still on the system.
The other possibility may be power. I'm not familiar with the new cards, but it may require 2 power plugs, and your PSU may only have one.
I hope I understood your problem correctly, and I wish you luck.
~J
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