Good question!
If you were to get someting in the area of 30-35 gallons, you would ultimately have enough room for 3 (
maybe 4 maximum) fancy goldfish. Or 2 common/comet type goldfish. Remember, when your talkng about goldfish, your talking about one of the heaviest producers of wastes. It is a common line of thought among sucessful goldfish keepers that more gallonage per goldie is better.
Heres a good list of stuff to start out a tank and get it completely ready for your new additions.
Hardware:
35gal Tank, Canopy, Light/s (flourescent)
Penguin 330 filter or 2 Penguin 170 filters (its beneficial to have two smaller filters than one big one in case one fails)
Thermometer and Heater
Master Test Kit (ammonia/nitrIte/nitrAte/ph/kh)
Air pump and stone
Dechlorinator/detoxifier like Prime or Amquel +
Variety of foods (check out goldfish feeding forum)
Ornaments and Gravel (silk plants and ornaments without sharp edges are recommended)
Just in case you arent familiar with the cycle of a fishtank, check this link out:
Cycling tanksAt the bottom of the cycling page, you'll find a link to "fishless cycling". I think youll agree that it is optimal to cycle your tank, without fish, completely before purchasing your new goldies. That way they don't have to deal with the stress of cycling a tank themselves.
There are several corresponding forums for each area of care. If you ever have any questions regarding anything in these areas, post your question there and your sure to get some very knowledgable advice.
Good luck and
Paul