Definitely agreed, reason. This is the minimum. I think fishcatch, your estimate may be the happiest choice for the goldfish, certainly, but if you're on a budget right now you could go with something a bit smaller, say a 40 gallon, and be alright in the short term.
I have apple snails (Pomacea bridgesii and Pomacea Canaliculata) with my goldfish, and they get along very well. Snails, you say, gross and boring. Not so! I am totally in love with my snails. They're interesting, and they have such personality! They jump off the side of the tanks and surf in the bubble wand, they interact with each other, they clean the tank... Mine are all about the size of a golf ball and the goldfish don't bother them at all. Check out applesnail.net. Bridgesii species are usually sold as your standard "mystery snail" in most stores. I have six in a 60 gallon. The maximum stocking recommended is about 3 gallons per snail, but fewer snails will be fine.