Thanks!

And is the difference between 1.35v and 1.5v (with 1.35v recommended for the T420) likely to be an issue? Most available ram seems to be 1.5v.

Soon to be a second time owner of a Thinkpad (the first one was in the beginning of the 1990s, so only a distant memory). It is a T420i with 4gb of ram, bought second hand (yet to arrive). My plan is to upgrade to 8gb using Crucial ram. I assume that the existing 4gb of ram are occupying only one me...