A-Tech 8GB (2 x 4GB) RAM for Lenovo THINKPAD THINKPAD X61S | DDR2 800MHz SODIMM PC2-6400 200-Pin Non-ECC Memory Upgrade Kit at Amazon.com
CMS 4GB (2X2GB) DDR2 5300 667MHZ Non ECC SODIMM Memory Ram Upgrade Compatible with IBM Lenovo® Thinkpad X61 Notebook Ddr2 Pc5300 - A37 at Amazon.com
X61 | Lenovo Wiki | Fandom
jonchoo: Lenovo ThinkPad X61 ultraportable review
RAM 8GB - Lenovo ThinkPad X1 Extreme Gen 3 (20TK)
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