I have 2 setups. One for fish and one for corals/inverts.
For fish having two 20gal long tanks (really 15gal) it makes it easy to do the tank transfer method or ttm for short. For these tanks i have a very simple setup that consists of a couple pvc pipes, heater, floating therm, an air stone, and a diy filter powered by an air pump.
If you are planning to house corals in one it needs to be a setup good enough to grow them and cycled. I use a red sea max130 for my coral. The nice thing is you dont need a ton of light in a smaller system. Mine has pc lighting but the corals are kept close to the bulbs and are getting good par.
Check out the stickies in the fish disease forum for more detailed info.