40-60w is a good range for a 20g. Is it a 20g high (24" long)? If so, the higher end of that range would be fine. You might can find some fluorescent fixtures at Home Depot, but you'd need a couple to get that much light.
I would go for power compact lighting or T5 HO.
This fixture (the 24" one if your tank is 24" long):
http://www.drsfostersmith.com/produc...0&pcatid=16770 would be perfect. You shouldn't need to inject co2 if you have a good bit of stem plants to take in nutrients. You'd have a wider variety of plant choices as well.
A 65w Compact Fluorescent fixture like this one:
http://www.drsfostersmith.com/produc...71&pcatid=9871 would work as well, but again, make sure you have a good bit of stem plants. You may need to inject co2 with this one....but DIY CO2 (yeast method) would work nicely.
A double fluorescent fixture like this:
http://www.drsfostersmith.com/produc...2&pcatid=13612 would give you 30 watts, which would be low light. You wouldn't need co2. The only downside is this fixture costs almost as much as the others, so you might as well pay $10 more and get more wattage.
If you can't order online, you can check out your LFS. They may have some fixtures for sale there. They may be more than online though.