Hmmm...well, you'll have to go what works for you, but here are some options:
Filter:
Whisper 30
Marineland 170 or 130 (depending on the fish you get)
AquaClear 200
Heaters:
Hagen Tronic 100watt submersible
Visi-Therm 100watt submersible
Air Pumps:
Tetratec 80
Profile 1500
Gravel:
approximately 25lbs any astectic color you want
Decor:
Anything you want
Fish:
what type of fish interest you?
If you think 20 gallons is big...oh boy, you've got to think again. Some fish, like cichlids and fast growers, definately need more space than 20 gallons, but you could probably get away with 12 smaller tropical community fish (tetras, sm barbs, mollies/swordtails/platys etc).
Currently my 20 gallon high is set up thus:
Filter: Marineland Penguine 330
Extra Media: 2 AquaClear 150 amrid pillows
Heater: None (temp stable 72oF)
Gravel: 40lbs Estee walnut colored gravel
Air Pump: TetraTec 80 (sitting on floor)
Airline and check valve
Air stone: Aqua Garden 12" bubble bar
Decor: An assortmant of nice green plastic plants by AG and PenPlex, a PenPlex replica tree stump, and some quartz stones I picked up from the beach and scrubbed and bleached and scrubbed...
Fish:
1 calico fantail goldfish (1/2 inch)
1 black telescope goldfish (1/2 inch)
1 red/white fantail goldfish (1/2 inch)
1 -Corydoras sterbai- (1 inch)
1 -Horabagrus braychysoma- (1 1/2 inch)
Water Changes: 6 gallons twice a week, gravel vac siphon
The two catfish are pretty new additions, not even a week yet, but they're doing fine and eating. I hope to get a pair more of each this week to give them some friends, since they're schooling species that need playmates (that and I'm getting tired of watching them pace the glass all the time, back and forth, back and forth, back and forth...).
Good luck with your tank
~~Colesea