A good memory? or a lot of good memories?

INCORRECT		CORRECT
	I have a good memory of last year's vacation.		I have a lot of good memories of last year's vacation.

"To have a good memory" means to be able to remember things well. "Good memories of," means to have "fond recollrections of."

The first sentence does not make sense as you generally either have or don't have a good momery. It is not generally used with one specific event. The second implies that I had fun last year and I remember the events.