I have done this successfully in an older Dell Precision Mobile Workstation and an Alienware. It isn't a difficult thing to do. Make sure to get replacement thermal pads to apply to the new GPU. Give it a shot.
So I put in nVidia and Dell doesn't want to run. I can hear two beeps and then one beep every few seconds. The screen is black. I noticed that there is an additional port: lvds_nb but connecting there does not give anything, and with Radeon on the screen changes only colors