What control maximum amount of memory

http://www.samintl.com/mem/n13053.htm

The maximum is 512MB

You only have two sockets in these machines.... so if they currently have two 128MB, all you can do is remove those two and replace them with two 256MB DIMMs to get the 512MB max.

The system BIOS, the chip-set, and how the chip-set is implemented on the motherboard control the maximum amount of memory that the system can address and access.

If the manufacture does not connect all the address lines, that can limit the maximum amount of RAM addressable.
Likewise, the BIOS can be written to only look for a specific maximum amount of RAM.

DIMM structure can also come into play.

I believe that there was another question on a similar e-machine where the user was actually able to use a 256MB DIMM in slot 1, and a 128MB in slot 2.
Even though e-machines stated that the maximum RAM for the PC was 256MB.
As I recall the specifics of the 2 DIMMs was critical to getting the extra memory recognized.

Chas