If you bought the wrong memory in the first place then yes it could be right but I'm puzzled as you say it ran perfectly for so long. That suggests to me that the ram was ok, if it wasn't it would have shown problems from day 1 or so I would have thought.
Are you sure you picked them up correctly? Perhaps they meant one of the ram modules has picked up a fault which can happen.
Regarding the multiple disk errors perhaps the hard drive is failing. What I tend to use to check that is the manufacturers software to check it eg MAXBLAST for a Maxtor drive etc.