CMS 8GB (2X4GB) DDR3 10600 1333MHZ Non ECC SODIMM Memory Ram Upgrade Compatible with Lenovo® Thinkpad X200 2024, 7454, 7455, 7457 - A29 at Amazon.com
REVIEW: Thinkpad X200 (durable beast, less than $100) - YouTube
OFFTEK 1GB Replacement Memory RAM Upgrade for IBM-Lenovo ThinkPad X200 Tablet (7450-xxx) (DDR3-8500) Laptop Memory at Amazon.com
NEW 8GB Kit 2X4GB DDR3 IBM Lenovo ThinkPad X200 X201 Laptop Memory RAM - Walmart.com
8GB 2 x 4GB DDR3 Memory for Lenovo ThinkPad X200 7454, 7455, 7457, 7458, 7459-xxx PC3-10600 1333MHz 204 pin SODIMM Laptop Notebook RAM (PARTS-QUICK Brand) at Amazon.com
X201, I think from about 2010 or 2011. It's a little slow, but with the ssd, 8gb ram (instead of 4) and running Linux Mint, it sure isn't bad for a 10+
Libreboot Thinkpad X200 SeaBIOS No Battery, w/ GRUB, 8GB RAM, 120GB SSD w/ Dock | eBay