The Fantom-X series uses off-the-shelf computer RAM:
3.3V DIMM (either 128MB, 256MB, or 512MB)
Must be either PC 100MHz or 133MHz speed
If you expand your Fantom's RAM, you'll also need to increase its storage capacity:
PC card, up to 1GB in size (inserted into the back of the keyboard where it says "PC CARD")
Alternatively, you may use a SmartMedia or Compact Flash card with a PC card adapter.
Memory and storage are both available at most computer parts supply stores.