Lighting really depends on what types of corals you want. If you want SPS, you'll want a high powered fixture something in the order of a 150 watt MH, or a 4 bulb T5HO fixture. If you are looking to keep mostly softies or some of the less demanding LPS a 2 bulb T5HO or even a larger Power compact fixture would work. I know for mushrooms and low light corals PC lights will do the job.
A 20g long will probably work to your advantage too since it is only 12 inches deep. Makes it easier for light to penetrate to the sand bed if you put corals there.
Do you have any corals in mind?