yeah i agree with trash and schitzo, goldfish do get HUGE, its a huge mistake that petstores make when they tell you about "goldfish bowls".... for a 20 gallon, here's the thread from what i'm planning to do in a few weeks with mine.... its all tropical fish instead, but just another idea for you to consider.... http://www.aquariacentral.com/forums/showthread.php?t=152260
If you get em when they are 2" you might be able to keep them in a 20g for about 2 months, then you would have to upgrade. If you had no plans of upgrading then dont bother getting one, they grow pretty fast.
Got mine when they were about 2-3" in around a years time they are 4x that.