• The Japanese site http://home.r03.itscom.net/forestar/PJ128M.htm shows several different upgrades, from 16MB up to 128MB (!!) internal RAM. Please stand by for an English translation. It may soon be available here.
• The article Another memory upgrade by Carlos Izzo Videla describes a piggyback upgrade from 2MB to 4MB or 6MB of a 200LX with 2MB on-board memory.
• The page http://www.upl.cs.wisc.edu/~orn/tech/200lx.html by Rudy Moore describes how to add 2MB to a 1MB or 2MB 200LX, ending up in 3MB or 4MB total memory.
  

For all upgrades involving a daughterboard, i.e. a RAM expansion module, you need a daughterboard socket (connector) on the motherboard. Some motherboards don´t have this socket, some do. 4MB 200LXs and 2MB 100LXs always have one. All 200LXs which don't have one installed have at least the soldering pads for such a connector. Here is the datasheet of the needed connector. You can buy such a connector from Thaddeus or Rundel. These models can be upgraded with a daughterboard: 100LX 2MB model, 200LX all models, 1000CX all models.

This is a 4MB upgrade daughterboard by Times2Tech which upgrades a palmtop with 1 MB on-board memory to 5 MB, a 2MB motherboard to 6 MB. This board is simply plugged into the RAM socket, no additional wires are needed:

This is a 8MB upgrade daughterboard by Times2Tech which upgrades every kind of basic configuration to 8 MB, because the on-board RAM is removed entirely:

This is a 32MB upgrade daughterboard by Times2Tech:

The following pictures show the additional wires which are necessary for the 32MB memory upgrade. Some pins of the CPU chip have to be removed from the mainboard and wires have to be attached to them. These are pins 24 and 25 from the right on the pin row which faces the battery compartment. The wires lead directly onto the memory upgrade board. Pin 24 (the right one) leads to the pad labeled "A1" of the Times2Tech 32MB upgrade board and pin 25 (the left one) needs to be connected to pad labeled "A10".
This is a very tricky job and not recommended to be done by people who are not experts!

The pictures below show the 64MB upgrade board. 3 additional address lines must be connected to the CPU for that kind of upgrade. The daughter board is much larger than the boards of the 32MB and smaller upgrades. There are also 64MB upgrades which have the same small daughterboard size as the other upgrades. Klick on an image to view a larger one with more details.