Goldfish grow pretty quickly, and its cruel to keep them in a small tank. The rule I've always been told is 20 gallons for the first goldfish, plus 10 additional gallons per extra fish after that. They're VERY messy fish (as far as eating and waste products) and they really need a lot of room. Definately don't move them to goldfish bowls, and if possible, get a bigger tank.
Even little tiny "feeder" goldfish can get very big -- 6 to 12".