Both X220 and X230 support up to 16 GiB RAM and are ideal for coreboot. The
main difference is that X230 has native USB 3.0 whereas X220 not.
Starting Haswell (4th generation Core), Intel implemented a treacherous
anti-feature called "Boot Guard". So it's impossible to change the firmware
on newer platforms with Boot Guard. One exception is the high-voltage Haswell
processors (without U or Y in their model names).
Stay away from: X240 or newer, T440/s or newer, P50 or newer. When you try to
modify the firmware, it will instantly power off (forcibly shut down by Boot
Guard).
