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
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
X61, still a great writing/ second machine after all these years : r/ thinkpad
My X61 Thinkpad, which I use for school. T8100 CPU and 1.5 GB of RAM, running Ubuntu Mate 18.04. : r/thinkpad
LENOVO THINKPAD X61 7673 TABLET USER MANUAL | ManualsLib