Thanks pjmelect, I really appreciate your reply.
I must admit, the instability was over the course of a week before one of them finally gave up! The 2nd went one day later.
They are of identical make/ model, and were bought at the same time, but they're only a year or so old.
I've not noticed any other problems with the PC at all though, but would I notice if the power supply was dodgy?
Again, thanks for you reply it is appreciated - I'm still wary of putting the new memory in ... ;-)