The information in this site is the result of my researches in the Internet and of my experiences. This information below is solely used for my purpose and may not be suitable for others.
The new video card installed on DELL Precision 3640 Tower is Nvidia Quadro P620, and the driver for this card is included in x11-drivers/nvidia-drivers. So, no need to download drivers from the NVIDIA website. Just run and follow the instruction of post installation at Gentoo Wiki: NVIDIA/nvidia-drivers :
# emerge --ask x11-drivers/nvidia-drivers