Kingston 2GB 256M x 64-Bit PC3-8500S 204-Pin SODIMM CL7. What does 256M means?

[Sxino]

I want to upgrade my memory capacity and replace 2 of this 2GB RAMs and buy 2 of 4GB.

The 2 x 4GB that I want to buy are described as:
Kingston 4GB DDR3 SO-DIMM PC3-10600 (1333Mhz) 204-Pins, 512M x 64, CL9, No ECC, 1.5V .

Does the 256M a value that match only with the laptop or I can get the new ones with 512M without problems?

And what does they mean?

 

[Barty1884]

16x 256MB = 4096MB (4GB)
https://www.kingston.com/dataSheets/KVR1066D3S7_4G.pdf

8x 512MB = 4096MB (4GB).
https://www.kingston.com/dataSheets/KVR13S9S8_4.pdf

Different ways to achieve the same thing.

Whether they'll pair perfectly with one another is another question. Mixing & matching can be a mixed bag in terms of success.
For guaranteed compatibility, the best route would be to buy a 2x4GB soDIMM kit (sold together in retail packaging).