nvidia GeForce 9800 GTX + on XPS 410 Question

Have you made the right connections to the card?  Depending on the brand, it will require one or two 6-pin PCIe connections for additional power.  The 7900 GS required one.  Double check to make sure the card is seated well in the slot and all connections are secure from the monitor to the video card.  The stock power supply should be enough for the card.  If you put the 7900 GS back in, can you get to the desktop?  If so, go into your Device Manager and see what it shows under your display adapter.  Disable anything that might be there.  You should not have to do this since you already had a video card running, but going from integrated graphics to a dedicated video card on some systems you have to do this so the video card will be recognized.  Otherwise, you get the black screen.  Follow the instructions HERE for a clean installation.

I cannot get onto my PC as of right now as it is being repaired by Dell.  I believe that the card that I just brought does have only one connection PCle six pin cord that connects to my motherboard though.