Often attributed to Gates in 1981. Although Gates has expressed similar opinions at other times [1], this particular quote has been denied by Gates:
I've said some stupid things and some wrong things, but not that. No one involved in computers would ever say that a certain amount of memory is enough for all time... I keep bumping into that silly quotation attributed to me that says 640K of memory is enough. There's never a citation; the quotation just floats like a rumor, repeated again and again.