Im going Saltwater

You'll need to go window shopping, see what you want, before we can really cover the full setup. Fish, coral, inverts?

Some basics: filtration. In a SW tank, live rock is one of the best options, simply because it doesn't require maintenance like a power filter will. Not sure what you mean by an index?

Lighting will vary with what you want to keep. Fish only, pretty much any lighting that pleases you will be fine. Corals and anenomes all need quality lighting, a minimum of 3 wpg, though more will be better. Anenomes are not for beginners--I'm pretty much against them in any setup, simply because they don't survive nearly long enough for most people, especially in new tanks.

Other equipment: powerheads for water movement, at least 10 gph. A protien skimmer isn't essential, but they are highly recommended, especially for reef setups. Hydrometer or refractomer is a must have, as well.

I knew you would go saltwater...... MUHAHAHAH!

Well, here's everything that's in my SW setups:

Tank. Heater. Powerheads (1 for every 2 foot of tank). Sand. Rock. Lights. Skimmer. I also run a sump on both tanks, but that's it. No power filters, no gizmos, nothing. Water, obviously, and the lights on the fish only are VHO fluorescents because I had them available, while the reef runs MH with VHO for actinic. Pretty simple. But--the rock is pricy, even cutting live rock with base rock.

For a 20, you could substitute a power head with a power filter that runs without media. PC would give you decent options if you want corals. Keep in mind that a 20 is going to be less stable than a larger tank--meaning you will have to par more attention to keep it in balance than a FW 20, and you'll be restricted to only a few small fish-still a variety to choose from, but you won't want more than 2-3 fish total.