Can't help with the hamster memory span query, but thought I'd clear up the goldfish myth.........
They apparently have a memory span of 3 months, and the goldfish bowl argument is, im sure, designed to stop us feeling guilty for shoving them in totally inadequate habitats.
......and I know that if you do anything to upset your cat it will never forget it. It will sulk for days and glare at you in the cruel way that only cats can glare.