Ok, actually I started memtest this morning and it's been running all day.
It has now gone through 11 passes and found 0 errors.
I have not yet done the step of running memtest on one stick at a time, but can do that next if you think it's advisable.
Would be grateful for any pointers on how to run memtest on one stick at a time. I've installed memory before (although not on this machine) but have not run memtest like that before. Pardon a very noob question, but I'm assuming I
1) turn laptop off
2) turn laptop over
3) unscrew plate
4) remove one memory stick
5) screw plate back on
6) run memtest again just as I did before?