Gladly an offer of compatible RAM modules can be easily found on ebay.
The specs of compatible RAM are DDR2-667 PC2-5300 200 pins SODIMM and DDR2-533 PC2-4200 200 pins SODIMM (2x1Gb is maximum for Inspiron 640m).
Naturally I chose the fastest one.
New Kingstone modules came after 2 weeks of waiting, and THAT IS very reasonable time to deliver from Hong-Kong to Moscow, Russia.
It took time to plug in 2 modules, cause Module A is not so easily replaced as Module B. The dock of Modules B is situated under the bottom of the laptop. To reach Module A one should remove hinge cover and the keyboard (btw some samsung laptops have both modules under the keyboard).
The disappointment came after both modules were installed.
Memory speed showed as 533 both in Windows and in Bios setup.
After some googling (you see it’s hard not to find needed info on the net today) the answer came out.
This laptop model is shipped with one of two processor types. The fastest one supports 667Mhz RAM and the other supports only 533Mhz. Pity that my relative had the second one.
So if you ever have to choose between compatible RAMs for your laptop first check if your CPU really supports desired RAM speed.