Another important advantage of ECC RAM is early detecting of malfunctioning memory. I've yet to hear from anyone that he's regularly running memtest. While cosmic rays might be rare event, bad memory stick is certainly possible and it'll produce much more errors than cosmic rays. Some people live with it for a long time, cursing their blue-screened OS or "access violation error" programs. It's hard to distinguish software bugs from hardware bugs. ECC will let you know about it immediately.
