Please provide more details. Do you mean you have a desktop PC with Intel graphics, and you want to upgrade to a PCIe Nvidia card? People do that all the time, so please tell us what problem you have run into.
Confirm the type of video card slot your have, probably PCIe.
Confirm the rating of your power supply.
Purchase a video card, and if necessary, a larger power supply.
Download the latest video driver direct from Nvidia or AMD.
Install the card and power supply if needed.
Boot into Windows.
Install the video driver.
Get a Radeon 5770 or 6770. Your Pentium D can't be running at 4 gig, unless you are overclocking it.
A higher-end video card will be held back by the CPU, although if you run at resolutions past 1280x1024 you may want a higher-end card that should allow yo to run at higher detail. Check the link in my sig for details on the cards.