Yep, the doc is correct:
MODULES MUST BE ORDERED AND INSTALLED IN PAIRS for Dual Channel mode.
If 4GB is installed, the recognized memory may be reduced to 3.5GB or less (depending on system configuration and memory allocation).
2GB (Removable) or
You need DDR2, 800MHz, Non-ECC, CL6, 1.8V, Unbuffered, SODIMM, 200-pin