2GB Memory RAM Upgrade for the Lenovo Thinkpad R61 Series, T60 Series, T61 Series, X60 Series and X61 Series Laptops (DDR2-667, PC2-5300, SODIMM) at Amazon.com
4GB 2X2GB Memory RAM for Lenovo ThinkPad X Series X61 (7674ES7) 200pin 667MHz PC2-5300 DDR2 SO-DIMM Black Diamond Memory Module Upgrade System Specific Memory - Newegg.com
X61, still a great writing/ second machine after all these years : r/ thinkpad
OFFTEK 2GB Replacement Memory RAM Upgrade for IBM-Lenovo ThinkPad X61 (7762-xxx) (DDR2-5300) Laptop Memory at Amazon.com
Best Buy: Lenovo 12" ThinkPad Notebook 2 GB Memory 80 GB Hard Drive Black X61-2.0C2-2-80-X-7P64