If you are increasing the amount of RAM installed -> On first power on after installing the RAM, system on boot up will complain about memory size changed, and to reseat the memory. Wait for about 60 - 90 seconds, and you will receive a few additional messages about F1 to continue or F2 for Setup. Go into Setup and verify that the system sees all memory installed. If installing 2 x 2GB DDR2, max possible, 4GB. BIOS setup should report 4GB installed, of which 3.37GB usable.
Nice guide! I just resurrected an old D620 that was on the pile, that I occasionally use for one piece of ancient code. I've disassembled, aired out the dust, and cleaned it. Replaced the non-functioning Dell battery with an OMCreate aftermarket battery.
This is a Core 2 Duo 2.3 GHz. Upgraded the HDD to a Hitatchi 320 GB SATA3 that was sitting here. Even though the D620 only has a SATA1 interface, might pick up a SSD, just to give it some more life. I did a clean install of Win7x64 bit + SP & patches (Nov2016), used the Dell's Vista nVidia NVS 110 video driver (File = R142931), installed and worked great.
I went ahead and ordered a replacement keyboard, since it has some sticky keys & a missing key cap. Also 2 x 2GB SODIMMs. That should give new life to this 10 year old notebook and keep it out of the landfill for at least another 4-10 years.